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put this solution on YOUR website!I'm assuming that the triangle is a right triangle and that side 3 is the hypotenuse.
We basically have this triangle set up:
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:

where a and b are the legs of the triangle and c is the hypotenuse

Plug in a=8, b=x, and c=12. Now lets solve for x

Square each individual term

Subtract 64 from both sides

Combine like terms

Take the square root of both sides

Simplify the square root
Which approximates to...
So our answer is