For this problem, you would use the Pythagorean theorem a^2 +b^2=c^2 . Given that the base is 12, that will be your A and your height will be B. The hypotenuse will be C. 12^2 + b^2= 15^ 144+b^2=225 -144 -144 B^2=81 (Take the square root of 81) b=9 m