Question 548697
You are asked to evaluate 
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{{{2x^2+3xy-4y^2 }}}
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when x = 2 and y = -4
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Start by evaluating {{{2x^2}}}
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When x = 2 then {{{2x^2 = 2*2^2}}}. This is equal to {{{2*2*2 = 8}}}
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Next evaluate {{{3xy}}}
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Substitute 2 for x and -4 for y to get:
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{{{3xy = 3*2*-4}}}. The 3*2 = 6 and then the 6*-4 = -24
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Finally, evaluate {{{4y^2}}} when y = -4. Substitute -4 for y to get:
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{{{4y^2 = 4*(-4)^2}}} and this becomes {{{4*(-4)*(-4) = 64}}}
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Now combine all three terms. Don't forget that this last one has a minus sign preceding it.
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{{{2x^2+3xy-4y^2 = 8 +( -24) - (64)}}}
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Remove the parentheses and then sum the terms on the right side to get the answer:
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{{{8 - 24 - 64}}} 
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Combine the +8 and -24 to get -16. Then add the -16 to the -64 to get -80.
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Hope this helps you to find where you were having trouble. In this case it could easily involve being confused by the signs.
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