Can you help me find the computation to input in mathematica to solve this c: If you did b correctly you got a list with one element which was a list of rules. Get the inner list and then substitute solutions into the expression for the distance between p and q.. You should get an expression which only involves a,b,c,x3,y3 . By the discussion of the distance between a point this is the distance between the point and the line. You should get Sqrt[(-y3-(b c+a b x3-a^2 y3)/(a^2+b^2))^2+(-x3-(a c-b^2 x3+a b y3)/(a^2+b^2))^2]