Question

If the triangles in the above figure are proportional, what's the length of side b?

Answer 1

The length of side b is 36 units.

9/b=90/360
3240=90b
36=b

Answer 2

We are give that the two triangles are proportional, this means that:
BC/EF = CA/FD

We are also given that:
BC = 9 units
EF = 90 units
CA = b (The value we want to calculate)
FD = 360 units

Substitute with the givens in the proportionality relation mentioned above to get the value of b as follows:
BC/EF = CA/FD
9/90 = b/360
b = (9/90) * 360
b = 36 units