Question

-y=15-4x divide to make y positive

Answer 1

-y= 15-4x
:-1 :-1 :-1
y= -15+ 4x

Answer 2

Simple. To change a variable/number's sign simply divide/multiply by a negative 1. So in this case divide both -y and 15-4x by a -1. To get your final answer you would flip the signs of the equation and since you divided by -1 only their signs will change. In the end you get $\frac{-y}{-1} = \frac{15-4x}{-1} or y=-15+4x$