Sketch a right
triangle having adjacent side(A) is given as “3”, hypotenuse side (H) is “x”
and assigning angle “a” as the angle between A and H. Using Pythagorean theorem,
you will get “square root of x-squared minus 9” as the opposite side (O). Using
SOH CAH TOA function, and since secant is the reciprocal of cosine, sec(a) =
x/3. Thus, a = arcsec(x/3). The remaining expression tan(a) is Opposite side
over Adjacent side which is equal to “square root of x^2 - 9” over "3". Therefore, the
algebraic expression would be: tan(arcsec(x/3)) = “sqrt (x^2 -9)” /3. Different answers can be made depending on which side you consider the “3” and “x”.
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 

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.
EACH central angle = 360 / number of sides
number of sides = 360 / each central angle
number of sides = 360 / 40
number of sides = 9