z= 3x + 5xy
move 3x to the other side
sing changes from +3x to -3x
z-3x=3x-3x+5xy
z-3x=5xy
divide both sides by 5x to get y by itself
z/5x-3x/5x=5xy/5x
cross out 5x and 5x
divide by 5 and x
then becomes 1*1*y=y
-3x/5x ( cross out x and x )
then becomes -3/5
Answer:
y=z/5x-3/5