So to solve this equation, you will want to isolate the variable, b. In order to do that, b has to be on its own on one side of an equal sign. So you have to get rid of everything else on that side, namely the -9. Since b is multiplied by -9, in order to get rid of it you will have to do the opposite, or divide it. But, whatever you do to one side of the equal sign, you have to do to the other; otherwise it changes the equation. So you have to divide both sides by -9 so that b will be on its own.
So.
-9b=90
-9b/-9 = 90/-9
b= -10
