onto mean : for every element in co-domain there should be a pre-image in domain right ??
now assume there's is an element in co-domain 'c'
c is an arbitrary element
now definitely element 'c' have pre-image in domain right ??
x = b - c / a
now a and b are const it's given in the question and a ! =0
for any value of c we get the pre-image in domain
you can check it now
C can be any real number
for every value in c we get pre-image in domain of the function