Question

F(x)=-x+3 what’s f(x)

Answer 1

f(x) is another way to say y. F(x) is most commonly used when comparing functions. f(x) is just y. Therefore, writing
f(x)=mx+b is the same thing as writing
y=mx+b

Answer 2

f(x) is another word for y-intercept, so imagine y=mx+b, find your slope and plug it into the equation you need to find.