To find the inverse of the function $f(x) = 2x + 5$:

1. Replace $f(x)$ with $y$: $y = 2x + 5$

2. Swap $x$ and $y$: $x = 2y + 5$

3. Solve for $y$: $x - 5 = 2y$ $y = \frac{x - 5}{2}$

So, the inverse function is: $f^{-1}(x) = \frac{x - 5}{2}$

For the second part:

If $f(-1) = 3$, then since $f$ is one-to-one, $f^{-1}(3) = -1$.