1)7.8а+2.3а-5а при а=6     2)2(0.3b+5)+1,4b при b =4       3)6(а+1.5)-2.8а при а =1.5                
4)4(b+0.8)+1.7b при b =2.3.

7.8a + 2.3a - 5a = (7.8 + 2.3 - 5)a = 5.1a5.1 * 6 = 30.62(0.3b + 5) + 1.4b = 2 * 0.3b + 2 * 5 + 1.4b = (0.6 + 1.4)b + 10 = 2b + 102 * 4 + 10 = 186(a + 1.5) - 2.8a = 6 * a + 6 * 1.5 - 2.8a = (6 - 2.8)a + 9 = 3.2a + 93.2 * 1.5 + 9 = 4.8 + 9 = 13.84(b + 0.8) + 1.7b = 4b + 4 * 0.8 + 1.7b = (4 + 1.7)b + 3.2 = 5.7b + 3.25.7 * 2.3 + 3.2 = 13.11 + 3.2 = 16.31