Answer
$\frac{2}{5}$
Work Step by Step
$sin^{2}t$ + $cos^{2}t$ = 1
sin t = $\frac{\sqrt {21}}{5}$
$(\frac{\sqrt {21}}{5})^{2}$ + $cos^{2}t$ = 1
$\frac{21}{25}$ + $cos^{2}t$ = 1
$cos^{2}t$ = 1 -$\frac{21}{25}$
$cos^{2}t$ = $\frac{4}{25}$
cos t = $\sqrt \frac{4}{25}$ = $\frac{2}{5}$
