b: Get a system of equations in x and y whose solution is the point q defined here . Now use Solve to solve this system. The rule(s) you get should only have a,b,c,x3,y3 on the right hand side(s). If you do this correctly you should get {{x->-((a c-b^2 x3+a b y3)/(a^2+b^2)),y->-((b c+a b x3-a^2 y3)/(a^2+b^2))}}