I suppose you know how to find the equation of the line when two points of the line are given or when the slope of the line and a point on the line are given.
a)find the equation of the lines of the sides of the triangle, from that derive the slopes of the required lines ( ). Now you have slope and a point of the line which enables you to find the equations of the equired lines.
b) same as the first one find the slope and a point which the line goes through
c) You can find two points which the required line crosses (vertex and the midpoint of the opposite side).