Answer
$7x^4-9x^2$
Work Step by Step
Recall the following rule: $\frac{a^{m}}{a^{n}}=a^{m-n}$
In order to solve this problem, use the rule above and cancel out common factors that are in both the numerator and the denominator of the fraction.
$=\dfrac{35x^8}{5x^4} - \dfrac{45x^6}{5x^4}
\\=7x^{8-4} - 9x^{6-4}
\\=7x^4-9x^2$