f(x) = 3x2 − 2x for x = 4.

Welcome to the forum Squaddy   

The answer you just gave is a REALLY good one.  Thank you  :)))

You name (as an answerer) will be in my next "End of Day Wrap" post   

I'd assume you want to find the value of y in the function f(x) when x=4.

This is simple, all you have to do is to put 4 in where the x's are.

$${f}{\left({\mathtt{x}}\right)} = {\mathtt{3}}{\mathtt{\,\times\,}}{{\mathtt{4}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{4}}$$

$${f}{\left({\mathtt{x}}\right)} = {\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{16}}{\mathtt{\,-\,}}{\mathtt{8}}$$

$${f}{\left({\mathtt{x}}\right)} = {\mathtt{48}}{\mathtt{\,-\,}}{\mathtt{8}}$$

$${f}{\left({\mathtt{x}}\right)} = {\mathtt{40}}$$

So in the function f(x), when the value of x is four the value of y would be 40.