24
Step-by-step explanation:
R = D + 12 (1)
D = B + 6 (2)
R = 2*B (3)
Put (3) into (1): 2*B = D + 12 (4)
Rewrite (2) as B = D - 6 and put this into (4): 2*(D - 6) = D + 12
So: 2D - 12 = D + 12
Subtract D from both sides and add 12 to both sides to get: D = 24
(not suree)