Question 118094
I'm assuming that the triangle is a right triangle and that side 3 is the hypotenuse.



We basically have this triangle set up:


{{{drawing(500,500,-0.5,2,-0.5,3.2,

line(0,0,0,3),
line(0,3,2,0),
line(2,0,0,0),
locate(-0.2,1.5,8),
locate(1,-0.2,x),
locate(1,2,12)
)}}}

Since we can see that the triangle has legs of 8 and x with a hypotenuse of 12, we can use Pythagoreans theorem to find the unknown side.



Pythagoreans theorem:


{{{a^2+b^2=c^2}}} where a and b are the legs of the triangle and c is the hypotenuse




{{{8^2+x^2=12^2}}}  Plug in a=8, b=x, and c=12. Now lets solve for x



{{{6 4 +  x  ^ 2 = 1 4 4}}} Square each individual term




{{{ x  ^ 2 = 1 4 4 - 6 4}}} Subtract 64 from both sides



{{{ x  ^ 2 = 8 0}}} Combine like terms



{{{s q r t (  x  ^ 2 ) = s q r t ( 8 0 )}}} Take the square root of both sides



{{{x=4*sqrt(5)}}} Simplify the square root



Which approximates to...

{{{x = 8 . 9 4 4 2 7 1 9 0 9 9 9 9 1 6}}}


So our answer is

{{{x = 8 . 9 4 4 2 7 1 9 0 9 9 9 9 1 6}}}