No, that's not it. The firm’s revenue rises from TR = P x Q = $7 x 20= $140 to TR = $6 x 30 = 180. Therefore, the change in TR is 40. To get MR you need to divide the change in TR by the change in Q (which is 40-30 = 10), so MR = 40/10 = 4.
Great job!