What proof method does the fllowing proof use? Theorem: If for two integers x and y,2x+y=15 then y=4. Proof: Assume x and y are two arbitrary integers. Now assume that y=4. Then because x is an integer we have: x2x2x2x2x+y=5.5=11=154=15y=15 Thus if 2x+y=15 then y=4. proof by contraposition proof by contradiction proof by counterexample direct proof