Question: how to do f(x) maths?
Answer: Now you say "f (x) = 2x + 3; find f (–1)" (pronounced as "f-of-x equals 2x plus three; find f-of-negative-one"). In either notation, you do exactly the same thing: you plug –1 in for x, multiply by the 2, and then add in the 3, simplifying to get a final value of +1.