#### Answer

$c \approx 23 \text{ cm} $

#### Work Step by Step

RECALL:
In a right triangle,
$\sin{A} = \dfrac{\text{length of opposite side}}{\text{length of hypotenuse}}$
$\cos{A} = \dfrac{\text{length of adjacent side}}{\text{length of hypotenuse}}$
$\tan{A} = \dfrac{\text{length of opposite side}}{\text{length of adjacent side}}$
Use the sine formula above to obtain:
$\sin{61^o} = \dfrac{20}{c}$
Cross-multiply to obtain:
$c \cdot \sin{61^o} = 20$
Divide $\sin{61^o}$ on both sides of the equation to obtain:
$c = \dfrac{20}{\sin{61^o}}$
Use a scientific calculator to obtain:
$c = 22.86708136
\\c \approx 23 \text{ cm} $