Answer:
41.68 or 42 (rounded)
Step-by-step explanation:
Presuming you are working on right triangles, you can use the Soh Cah Toa theorems.
Because angle A = 21 and side a = 16 you can use Toa or [tex] A=\frac{a}{x}[/tex]
[tex] 21=\frac{16}{x}[/tex]
Plugged into a calculator, you get 41.68 and some change.
If this is NOT a right triangle question, please let me know so that I can revise my answer.
