Answer
-$\frac{9}{20}$
Work Step by Step
We need a common denominator (bottom of the fraction) to subtract the numerators (tops of the fractions). Thus, we multiply $\frac{3}{4}$ by $\frac{5}{5}$ to get $\frac{15}{20}$ and multiply $\frac{3}{10}$ by $\frac{2}{2}$ to get $\frac{6}{20}$. (This is legal because $\frac{2}{2}$ and $\frac{5}{5}$ are equal to one).
We now have:
$\frac{6}{20}$-$\frac{15}{20}$=-$\frac{9}{20}$
