Solving equations 3 and 4: From 3, $c = 7 - 3d$. Substitute into 4: $2(7 - 3d) + 4d = 8 \Rightarrow 14 - 6d + 4d = 8 \Rightarrow -2d = -6 \Rightarrow d = 3$, then $c = 7 - 9 = -2$.