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a) 40 a - 15 = 28 - 21 a
⇔ 40 a + 21 a = 28 + 15
⇔ 61 a = 43
⇔ a = 43/61
b ) 35 x - 21 = 18 x
⇔ 35 x - 18 x = 21
⇔ 17 x = 21
⇔ x = 21/17
c) 20 x - 28 = 21 - 28 x
⇔ 20 x + 28 x = 21 + 28
⇔ 48 x = 49
⇔ x = 49/48
d) 4 - 21 - 63 x = x
⇔ - 63 x - x = - 4 + 21
⇔ - 64 x = 17
⇔ x = - 17/64
e ) 4 x - 6 x - 30 = 100
⇔ - 2 x = 100 + 30
⇔ - 2 x = 130
⇔ x = - 65