Question

.

Answer 1

pythagorean identity is
\[\sin ^{2}x+\cos ^{2}x=1\]
now sinx=21/29
so use the theorem :)

Answer 2

\[\cos ^{2}x=1-\sin ^{2}x=1-(\frac{ 21 }{ 29})^{2}=400/841\]
so
\[cosx=\sqrt{\frac{ 400 }{ 841 }}=+-20/29\]