The distance from a point to a line is the shortest possible distance, which means that it's the length of the segment between the point and line that's perpendicular to the line.
Perpendicular things have negative, inverse slopes...
And we know how to find the equation of a line from a point and slope.
Given two line equations, the point where they cross is just the point where both are true, so solve them as a system of equations to get the end point
Finally, find the distance between the point and the intersection.