If you have an equation for f(x), like

f(x) = 4x + 3

Then if you are asked for f(a), that means to substitute
a for x.

f(a) = 4a + 3

If there is a number in the parentheses like 2, that is,
if you are asked for f(2), you substitute 2 for x, and simplify:

f(x) = 4x + 3
f(2) = 4(2) + 3
f(2) = 8 + 3
f(2) = 11

If there is an expression like y+2 in the parentheses, that
is if you are asked for f(y+2) you substitute y+2 for x, then
simplify:

f(x) = 4x + 3
f(y+2) = 4(y+2) + 3
f(y+2) = 4y+8 + 3
f(y+2) = 4y+11

The left side stays f(y+2). It does not mean "fy+2f".  It looks like
multiplication but it is not. 

You always substitute what is in the parentheses for x and simplify,
if possible.

Sorry we can't figure out what you are asking.  If you'll copy it
word-for-word exactly like it's stated in your book or computer
page, we can help you.  If you type it just as it is given in the 
book in the thank-you note form below, we'll get back to you by email.

Edwin