how do I solve for x ?

x = 4
Hi there !
use Thales theorem
x/(x + 8) = 4/12
12x = 4x + 32
12x - 4x = 32
8x = 32
x = 32 : 4
x = 4
4/(8 + 4) = 4/12
4/12 = 4/12 (true)
Good luck !