Question

If f(×)=4×-3, what is f(×)

Answer 1

f(x) is the function, that is, when x take some value, f(x) is the image for this value, so you name y = f(x), then:

if x = a

y = f(a) 
y = 4a - 3


If y = b

y = f(b)
y = 4b - 3

So in this case y = f(x) represents a straight line

Answer 2

f(x) is another way of saying y.