What is the value of b in this diagram?

Apparently, you are to make use of the law of sines. It tells you

  b/sin(B) = a/sin(A)

You can multiply by sin(B) to find b.

  b = sin(B)·a/sin(A)

Substituting given values, this becomes

  b = sin(40°)·7.89/sin(121.5°)

  b ≈ 5.95

The Law of Sines looks like this:

b a

=

sin B sin A

Here we know the values of a, A and B:

b 7.89

=

sin 40 sin 121.5

7.89*sin 40 deg

Cross-multiplying, we get b =

sin 121.5 deg

answer: f = 32.2m

step-by-step explanation: that is the answer