Vectors (Perpendicular vectors) problem

Position vec of points A and B with respect to origin O are 2i + 4j and 6i + 7 J

question:- position vector of point C is 3i-4j. find the vector equation for the line OC and show OC is right angle to AB.

and also - Find the position vector of point of intersection of OC and Ab and hence find the perpendicular distance from O to AB.

i did r= 0 (because its the origin) + u(3i-4j) , where u is a scalar quantity..i don't know how to show its right angles with AB and also the following parts.