-10(3-4x) + 51 = 7(5x+3)

-30 + 40x + 51 = 35x + 21

40x + 21 = 35x + 21

40x - 35x = 21 - 21

5x = 0

x = 0

Проверка:

-10(3-40) + 51 = 7(5*0+3)

-10(3) + 51 = 7*3

-30 + 51 = 21

21 = 21

Ответ: x = 0.