This below worksheet help you to understand how to compute perpendicular distance
Perpendicular Distance Formula =
Coordinates (X1,Y1) = (1,2)
Ax + By + C=0 ---> 1X + 2Y + 3
Perpendicular Distance = ((1x1) + (2 x 2) + 3)/√(1^2 + 2^2)
Hence the perpendicular distance is 3.5777
When you try such calculations on your own, this perpendicular distance alculator can be used to verify your results.