Draw to show how you can use models to compare 345 and 391

You can draw 3 wholes 4 tens and 5 ones. For 391 you can draw 3 wholes 9 tens and 1 ones.

The inverse is the symmetrical function relatively to the function y=x. So, it basically means, that to find the inverse, you just need:
1. Switch x and y (or f(x)): x = 5H(x) + 2
2. Solve for y again (or for f(x)):
5H(x) = x -2
H(x) = (x-2)/5
This is the inverse.