Plug in to solve this one. (Note, it doesn't matter if the equation doesn't work with the numbers you pick, since you're not asked to solve. This is just to help you see how the equation should go.) Say the shortest side, x, is 6. Then the other two sides are 8 and 10. So the equation you'd set up is:
6^{2} + 8^{2} = 10^{2}
Or, in terms of x:
x^{2} + (x + 2)^{2} = (x + 4)^{2}
That's choice (C).