Let's start:
[tex]5(3x^2+?x-2)=15x^2-20x-10[/tex]To find the coefficient missing here, you simply have to use ''multiply and divide property of equality'' and ''add and subtract property of equality'':
[tex]\begin{gathered} 15x^2+5?x-10=15x^2-20x-10 \\ \text{the same terms on the both sides, so you can eliminate them , then:} \\ 5\text{?x}=-20x \\ ?=\frac{-20x}{5x} \\ ?=-4 \end{gathered}[/tex]