Answer
$\frac{a^{4}}{b^{2}}$
Work Step by Step
The exponents count how many times the variable a is used as a factor in the numerator, 4 times, and how many times b is used as a factor in the denominator, 2 times:
$$\frac{a\times a\times a\times a}{b\times b}=\frac{a^{4}}{b^{2}}.$$
