[tex]x-Jane\\2x-John\\\\x-4-Jane\ (4\ years\ ago)\\2x-4-John\ (4\ years\ ago)\\\\2x-4=3(x-4)\\2x-4=3x-12\ \ \ /+4\\2x-4+4=3x-12+4\\2x=3x-8\ \ \ /-3x\\2x-3x=3x-3x-8\\-x=-8\ \ \ /\cdot(-1)\\x=8\\\\2x=2\cdot8=16\\\\\\Answer:John's\ current\ age\ is\ 16\ years.[/tex]