The two triangles are similar. The two marked sides 16 and 2x-6 are connected in such a way that the longer side (16) is exactly twice as long as the shorter side (2x-6)
In other words
longer side = 2*(shorter side)
16 = 2*(2x-6)
Let's solve for x
16 = 2*(2x-6)
16 = 2*2x+2(-6)
16 = 4x - 12
4x - 12 = 16
4x - 12+12 = 16+12
4x = 28
4x/4 = 28/4
x = 7 which is the answer
