To solve the expression
[tex]\frac{n}{-2.25}=15.6[/tex]Note that the variable "n" is divided by "-2.25", so to determine the value of n you have to cancel said division. To cancel the division you have to multiply the term by the same number, that is "-2.25". And, to keep the equality valid, any operation done to one side of the equal sign has to be done to the other side, so you have to multiply 15.6 by -2.25 too
[tex]\begin{gathered} -2.25\cdot(\frac{n}{-2.25})=(-2.25)\cdot15.6 \\ n=-35.1 \end{gathered}[/tex]