35,x, and (x+15) what is the equation

Statement: if two distinct planes intersect, then their intersection is a line. which geometry term does the statement represent? defined term postulate theorem undefined term

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Solve the following simultaneous equation by using an algebraic method (either substitution or elimination) 2x + 3y=-4 4x-y=11