According to the function g=ac +bc' when c=0 ,data on line b will be selected and if c=1 data on ine a will be seected .
so ,g= E*x' + 0*x=E*x'
f =y'x+y(output of 1st mux g)
=(xy' + Ex'y)
note; the g function is given to determine the 0th and 1th line of the mux.