Пошаговое объяснение:
1)
4a+3b=5
3b=5-4a
b=(5-4a)/3
2)
– (5.3a + 2.2b) + (1.7b – 0.7a) – (1.4b – 5.3a) = -5.3a-2.2b+1.7b-0.7a-1.4b+5.3a = -0.7a-1.9b
3)
2(2a-9)-3(5c+7) = 4a-36-15c-21 = 4a-15c-39 = 4 * 9/2 - 15 * 4/15 - 39 = 2*9 - 4 -39 = -25