The solution is r= 7/a-x How did i do this?
Your equation is ax+ r=7, so the goal is to isolate the r. You begin by dividing the a from the ax. Remember, what you do to one side, you must do to the other. So you do 7/a, since you can't simplify 7/a, leave it as it is. Next, your equation should look like this x+r=7/a. In order to isolate the r, you move the x to the other side by subtracting, which leaves you with the answer of r= 7/a-x.
Hope this helped!