7,8а+2,3а-5а = a(7,8+2,3-5) = 5,1a
5,1*6 = 30,6
26(а+1,5)-2,8а = 26a+39-2,8a = 23,2a+39
23,2*1,5+39 = 34,8+39 = 73,8