This problem claims that both triangles are similar because they are the exact same right triangle except one of them is bigger in area than the other.
The sides on the bigger triangle that you do know are 68 and 32. You divide those numbers by their match on the smaller triangle to find the scale factor, or how much bigger one triangle is than the other. For example, 68/17=4. This means that each of the larger triangles sides are divisible by 4.
Therefore, the side missing the x value will be equal to 60 because 15*4=60. You would then write the equation 4x+12=60. Subtract twelve from both sides to isolate the variable, so you would get 4x=48. Divide each side by 4, and then you will get your answer of x=12.
You can check the answer by plugging twelve into your equation as x, and you will get 60.