Question 181640
Hi, Hope I can help,
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First you would draw something sort of like this, you can make it more creative, and draw the actual pole
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{{{ drawing ( 300, 300, -10, 10, -10, 10, triangle ( -4, -2, 4, -2, 4, 8 ), locate     ( 5,4, 12), locate (0,-3,8), locate (0, 6, c))}}}
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It will look like a right triangle. 
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We are trying to find "c", Since we know that it is a right triangle, we can use the Pythagorean Theorem, which will work only with right triangles
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Pythagorean Theorem = {{{ a^2 + b^2 = c^2 }}}
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"a" and "b" refer to the legs of the triangle ( the horizontal and vertical lines ). I usually think of "a" as the vertical line, and "b" with the horizontal line, but it doesn't really matter.
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All you do now is replace the variables with the numbers we know
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{{{ a^2 + b^2 = c^2 }}} = {{{ (12)^2 + (8)^2 = c^2 }}} = {{{ (12)(12) + (8)(8) = c^2 }}} = {{{ 144 + 64 = c^2 }}} = {{{ 208 = c^2 }}}, to find "c" we will take the square root of both sides
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{{{ 208 = c^2 }}} = {{{ sq rt(208) = sq rt(c^2) }}} = {{{ sqrt( 208 ) = c }}} = {{{ c = + sqrt(208)  }}} or {{{ c = - sqrt(208)  }}}, a measurement cannot be negative so {{{ c = + sqrt(208)  }}}
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If you wanted to be most accurate, you would simplify your answer {{{ sqrt (208) }}}
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You can sometimes simplify radicals if you can find factors of the number that has a whole number as a square root, in this case we can take "16" out of "208"
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{{{ sqrt (208) }}} = {{{ sqrt (16) ( sqrt (13)) }}}({{{16(13)=208}}})
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{{{ sqrt (16) ( sqrt (13)) }}} = {{{ 4 (sqrt (13)) }}}, this is the simplified form

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If you rounded your answer, it would be 14.42 ft
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You can round your answer, you need to if they asked you to, but to be the most accurate, you would say the guy wire is {{{ 4 (sqrt(13)) }}} feet long, which would be your answer
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{{{ drawing ( 300, 300, -10, 10, -10, 10, triangle ( -4, -2, 4, -2, 4, 8 ), locate     ( 5,4, 12), locate (0,-3,8), locate (-3, 6, 4 (sqrt (13))))}}}
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Hope I helped, Levi