Don't know why they'd encourage a "guess-and-check" method.
But let's play their game.
Choose x=2 (I just invented it): Is 40(2) = 30(2+2)? Is 80 = 120? No.
Try x=1, x=3, etc.
Better yet, do the problem algebraically: 40x = 30x + 60, or 10x = 60.
Then x=6. Check: Is 40(6) = 30(6+2)? Is 240 = 30(8)? YES.
Answer: x=6
