Question

What is the equation of the exponential graph shown?

Answer 1

Do you know the equation for the exponential?
$$
y=f(x)=Ke^{-x}\\
f(0)=K=100\\
y=100e^{-x}
$$
Does this make sense?

Answer 2

it looks vaguely familiar

Answer 3

Here is a plot of the function using this equation:
http://www.wolframalpha.com/input/?i=+plot+y%3D100*e%5E-x%2C0%3C%3Dx%3C%3D8

Answer 4

Note that f(0) is the value of the function at x=0. This value is 100, so the exponential is really one of the easiest functions to create. f(0) is also the value of K in this equation. So K represents the initial value!