The answer is c = 3r. Note that c is the output and r is the input. I figured this out by using mental math and noticing the pattern. Each value the input is multiplied by 3, which results in the output.
(INPUT)2 * 3 = 6 (OUTPUT)
(INPUT)4 * 3 = 12 (OUTPUT)
(INPUT)7 * 3 = 21 (OUTPUT)
(INPUT)10 * 3 = 30 (OUTPUT)
(INPUT)12 * 3 = 36 (OUTPUT)
