Because the answer doesn't contain c or d, we must eliminate these between the two equations. Divide the first equation by d on both sides: cx/d=y, then divide both sides by x: c/d=y/x. Do the same with the other equation with d and z: c/d=z/z (ignore the fact that the z's may cancel out). Substitute c/d=z/z and we get z/z=y/x. Multiply both sides first by x: xz/z=y then by z: xz=yz.