б) (2,1-0,7x):0.48-3,5.

(2.1 - 0.7x) / 0.48 = 3.5
2.1 - 0.7x = 3.5 * 0.48
2.1 - 0.7x = 1.68
0.7x = 2.1 - 1.68
0.7x = 0.42
x = 0.42 / 0.7 = 0.6