Im still confused on how you got the answer to the question

Using trig ratios: sine, cosine, and tangent, you set up equations based on the parts of the triangles that are given or needed.
sin = opp/hyp
cos = adj/hyp
tan= opp/adj

Based on the picture a is the hypotenuse, b is the opposite, and 17 is the adjacent side in relation to angle 36 degrees.

To find a, which is the hyp and we have the adjacent, we will use cosine.
cos 36 = 17/a
to solve multiply both sides by a.
a* cos36 = 17
divide both sides by cos 36
a = 21

To find b, which is opposite, and we have adjacent, we will use tangent.
tan 36 = b/17
multiply both sides by 17
17 * tan 36 = b
b = 12.4