help!! help!! help!! help!! help!! help!! help!! help!! help!!

a/5 - b/2 = 10...(1)
2a-5b/10 = 10
2a-5b=10×10
2a-5b=100...(2)
a-b=29...(3)
2a-2b=58...(4)
subtract (4) from (2)
2a-5b=100
2a-4b=58
-b = 42
b = -42
2a-2b=58
2a-2(-42)=58
2a+84=58
2a=-26
a= -13