To find cosine, you can use one of the formulas in the Pythagorean Identity, which is Sin^2 + Cos^2 = 1. Using this information, we can plug in sin into this equation.
So, (21/29)^2 + cos^2 = 1
(21/29)^2 = 441/841.
Bring this over to the other side by subtracting it by both sides, so
cos^2 = 1 - 441/841.
To subtract, convert the one to have the same denominator and then subtract, so
cos^2 = 841/841 - 441/841 = cos^2 = 400/841 (square root this)
cos = 20/29 that is your final answer.