Since the triangles are similar, all sides are proportional by the same scale factor.
The relationship between the bases 32 and 8 is 4 (32/8 = 4). This means that the smaller triangle need to be multiplied by a scale factor 4 to result in the bigger triangle.
This means that 4x + 12 is 4 times greater than 15.
The equation to solve for x can be written as:
4x + 12 = 4*15
Now just solve for x as u normally would
4x + 12 = 60
(Subtract twelve from both sides)
4x = 48
(Divide both sides by 4 to isolate x)
x = 12
I hope this somewhat helped :)
Was my explanation clear enough?