For this problem, we can set up a proportion using similarity ratios. We know that triangle DEF is similar to triangle PQR, so we know that DE/PQ = EF/QR = FD/RP:
DE/PQ = EF/QR
3/(5x - 25) = 7/(6x + 4)
Cross multiplying, we get:
7(5x - 25) = 3(6x + 4)
35x - 175 = 18x + 12
Now, we can solve for x:
35x - 18x = 12 + 175
17x = 187
x = 11
So, x is 11.