a) $$a + 2a = 3a$$
б) $$25x - 19x = 6x$$
в) $$18b \cdot 2 \cdot 5 = 18b \cdot 10 = 180b$$
г) $$5 \cdot (3t - t) = 5 \cdot 2t = 10t$$
д) $$(21s + 79) \cdot 3 = 63s + 237$$