There is a long way to get the result - and of course a shortcut.

A. Long way:

- Calculate the equation of the perpendicular line to the line L:
- Calculate the coordinates of the point of intersection of L and the normal to L:
- Calculate the distance

B. Shortcut:

Let denote a point and a straight line then the distance d of P from L is calculated by:

With your problem: The line

Plug in the values of A, B, C and the coordinates of P and you'll get the same result.

I'll leave the second example for you.