The 'answer' is the number that 'r' must be in order to make the equation a true statement. There's only one number that can do it. Here's how to find it:
Copy the equation: 3.5r = 3(4 + .5r)
Expand the right side: 3.5r = 12 + 1.5r
Subtract 1.5r from each side: 2r = 12
Divide each side by 2 : r = 6