check the picture below.
so, when we have a line from a 90° angle running perpendicular to the opposite side, like in this case, we end up with three similar triangles, a large one containing the other two, a medium and a small one.
now, using the sides proportions as you see there, of the small and large one, we get that much.
Step-by-step explanation:
3x+12=45
3x=45-12
3x=33
3x/3=33/3
x=11
4.6 is the correct answer. Hope I helped:)