630 : (62*x - 42*x) + 53 = 68,
630 : 20*x + 53 = 68,
630 : 20*x = 68 - 53,
630 : 20*x = 15,
20*х = 630 : 15,
20*х = 42,
х = 42 : 20,
х = 2 1/10 (или 2,1),
проверка:
630 : (62*2,1 - 42*2,1) + 53 = 68,
630 : (130,2 - 88,2) + 53 = 68,
630 : 42 + 53 = 68,
15 + 53 = 68,
68 = 68