Samuel started out with 21 mowed lawns. 7(months) x 3(lawns) = 21
If he mows 3 lawns per month, then he would have to mow lawns for another 3 months. 3 x 3 = 9.
21 + 9 = 30
Although this is one mowed lawn over the objective, any less would not reach 29.
I'm not really sure what the question is asking, so here's two answers:
He would have to mow lawns for another 3 months in order to mow 29 lawns.
Or, he would have to mow 8 more lawns.