Answer
$a \approx 18$ 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 tangent formula to obtain:
$\tan{A} = \dfrac{\text{length of opposite side}}{\text{length of adjacent side}}
\\\tan{61^o}=\dfrac{a}{b}
\\\tan{61^o} = \dfrac{a}{10}$
Multiply 10 to both sides of the equation to obtain:
$\tan{61^o} \cdot 10 = a
\\18.04047755 = a
\\18 \approx a$
Thus, $a \approx 18$ cm.