If you have a right triangle and the side AC is the hypotenuse (the longest side) then your equation would be a^2+b^2=c^2 like mentioned before. but you have to put the numbers in the correct spot.
a in the equation= side ab, b in the equation= side bc, and c in the equation= side as
so..
3^2+b^2=7^2
9+b^2=49
b^2=40
b= the square root of 40 or
2 square roots of 10 (2 then square root sign with 10 in the square root) which would probably be the simplest form.
