(+ 2 (+ (* 3 (/ 12 4)) 4))
Working from the inside out: / 12 4 = 3
(+ 2 (+ (* 3 3) 4))
Then: * 3 3 = 9
(+ 2 (+ 9 4))
And: + 9 4 = 13
(+ 2 13)
Finally: + 2 13 = 15
This is in "prefix notation" or "Polish notation".