0.3(5х-7)=3(0.2х+3.2)

Найдем корень.

0.3 * (5 * х - 7) = 3 * (0.2 * х + 3.2); 

Скобки раскроем. 

0.3 * x * 5 - 0.3 * 7 = 3 * 0.2 * x + 3 * 3.2; 

1.5 * x - 2.1 = 0.6 * x + 9.6; 

Умножим на 10. 

1,5 * 10 * x - 2.1 * 10 = 0.6 * 10 * x + 9.6 * 10; 

15 * x - 21 = 6 * x + 96; 

15 * x - 6 * x = 96 + 21; 

x * (15 - 6) = 110 + 7; 

9 * x = 117; 

x = 117/9; 

x = 90/9 + 27/9; 

x = 10 + 3; 

x = 13.