What is the exponent of 0.00000072
The number 0.00000072 has no exponent.
My best guess is that this question is about scientific notation. Every non-zero real number can be written in the form $\pm d_0.d_1d_2d_3 \ldots \times 10^e$, where $d_0 \neq 0$. Here $e$ is the exponent. Some examples:

$32 = 3.2 \times 10^1$. The exponent is $1$.
$-0.0014 = -1.4 \times 10^{-3}$. The exponent is $-3$.
$3.14159 = 3.14159 \times 10^0$. The exponent is $0$.

Hopefully now you can solve the question on your own.