$\left(\dfrac{3}{4}\right)^2=\dfrac{3}{4}\cdot \dfrac{3}{4}=\dfrac{9}{16}$

$\begin{vmatrix}a&b\\c&d\end{vmatrix}=ad-bc$

$\begin{vmatrix} 4 & -5 \\ 3 & 2{,}5 \end{vmatrix}=4\cdot 2{,}5-(-5)\cdot 3=10+15=25$