To begin this question, we want to isolate the term containing the s, i.e. have the term containing the s by itself on one side of the equality. We do this by subtracting 8 from both sides. 4st = 5r - 8 To make s the subject of this equation, we want to undo whatever has been done to it. We notice that the s has been multiplied by 4 and by t, so, to counteract this, we divide both sides by 4 and t. s = (5r - 8) / 4t