There seems to be something missing. Either a missing diagram or more context to the problem, or perhaps more instructions. I'm not exactly sure.
Anyways, if you simply need to solve the equation for x, then...
3x+7 = 5x-9
3x+7-3x = 5x-9-3x ... subtract 3x from both sides
7 = 2x-9
7+9 = 2x-9+9 ... add 9 to both sides
16 = 2x
2x = 16
2x/2 = 16/2 ... divide both sides by 2
x = 8
I'm assuming after this point, you'll use x somehow to determine the length of segment AD. Unfortunately there isn't enough info provided to do that, but I'll let you take over from here.