You can put this solution on YOUR website! Okay, Mom, I have five kids, I can dig it...
From 75 = -5a/3, let us multiply both sides by -3/5 (the reciprocal) and get
(-3/5)(75) = (-3/5)(-5a/3)
That way the fractions on the right cancel out, leaving only a. Thus
a = (-3/5)(75) = -45