If a−6b+6c=4a-6b+6c=4a−6b+6c=4 and 6a+3b−3c=506a+3b-3c=506a+3b−3c=50, where a, b and c are real numbers, the value of 2a+3b−3c2a+3b-3c2a+3b−3c is
20
14
18
15