5x-10y = 90 

Since you want to solve for y, get all terms off the left side 
except the -10y.  You want to get rid of the 5x term on the left.

To do that, add -5x to both sides:

  5x-10y = 90
 -5x         -5x
----------------
    -10y = 90-5x

Divide every number on both sides by -10

   

Simplify:

   

   

Since you might want it in slope-intecept form y = mx+b you might
want to switch the two terms on the right

   

Edwin