Finding the zero's

Make sure you set the equation = 0 to find the zeros...don't plug in zero!  That is entirely different...

If you are asked to find the zero of f(x) = 2x + 5

2x + 5 = 0...then solve for x.
  x = -5/2

if you are asked f(x) = 2x + 5; f(0), then plug in zero...2(0) + 5 = 5

f(0) = 5, the zero of the function is -5/2