F4 Math一問~~~~~

Let y=2x
So, x=y/2
f(y/2)=20(y/2)^2-6(y/2)+3
f(y/2)=20(y^2/4)-3y+3
f(y/2)=5y^2-3y+3
So, f(x)=5x^2-3x+3
f(1/2)=5(1/2)^2-3(1/2)+3
f(1/2)=5(1/4)-3/2+3
f(1/2)=5/4-6/4+12/4
f(1/2)=11/4