GRE Sample Test 5 Quantitative Reasoning 2

To isolate a, divide both sides of the equation by 16 to arrive at:
a = $\frac{{64}^{\mathit{b}}}{16}$
Next, express the bases in base-2. 64 = $2^6$, so the numerator = ${\left(26\right)}^{\mathit{b}}$ = $2^6\mathit{b}$. Expressed in base-2, the denominator = $2^4$. So the equation simplifies to:
a = $\frac{\left(2^{6\mathit{b}}\right)}{2^4}$
Since we’re dividing terms with the same base, we’ll keep the base and subtract the exponents to arrive at: $2^{\left(6\mathit{b}-4\right)}$.