The y that you defined using an delayed evaluation is not a function (it has no arguments). Define it like this instead:

y[x_] := x^2

(As an aside, your parameters a and b do not need to be defined using a delayed evaluation.)

Although Mathematica has some elements inherited from Lisp, its syntax is not the same. So guessing proper syntax is likely to elad to all sorts of problems. A quick course on things for folks coming from other languages is here and can be helpful in getting you jumpstarted in using Mathematica and divorcing you from pre-suppositions when thinking of other languages that you know:

http://www.wolfram.com/language/fast-introduction-for-programmers/interactive-usage/