a-5b=7
a) x+3a-15b=29
x+3(a-5b)=29
x+3*7=29
x+21=29
x=29-27
x=2
b) x-123+5a-25b=137
x-123+5(a-5b)=137
x-123+5*7=137
x-123+35=137
x-123=137-35
x-123=102
x=102+123
x=225
c) x-2a+10b+17=4
x-2(a+5b)+17=4
x-2*7+17=4
x-14+17=4
x+3=4
x=4-3
x=1
d) 47-x+7a-35b=2a-10b+4
47-x+7(a-5b)=2(a-5b)+4
47-x+7*7=2*7+4
47-x+49=14+4
47-x+49=18
96-x=18
x=96-18
x=78