Answer
$n=\dfrac{x_1+x_2+x_3}{A}$
Work Step by Step
Multiply $n$ to both sides to find:
$n \cdot A = n \cdot \dfrac{x_1+x_2+x_3}{n}
\\nA = x_1+x_2+x_3$
Divide by $A$ on both sides to find:
$\dfrac{nA}{A} = \dfrac{x_1+x_2+x_3}{A}
\\n=\dfrac{x_1+x_2+x_3}{A}$
