(a+112+30):2*6-300=6
(a+112+30):2*6 = 300 + 6 = 306
(a+112+30):2 = 306 : 6 = 51
(a+112+30) = 51 x 2 = 102
a + 142 = 102
a = 102 - 142 = -40
