Once you have the equations for two lines, the point of intersection (a,b) satisfies both equations ....so if you equate the line equations
you can solve for the point x, which is 'a'
2x-10 = -4x+36
6x = 46
x = 46/6 = 'a' now use this value in one of the equations to calculate y (which is the 'b' value) .....then add them together to answer your Q