Which number can replace [[$ x $]]?

a) [[$ x(a + b) = 4a + 4b $]]

[[$ x = $]]

b) [[$ x(a - 3b) = -3a + 9b $]]

[[$ x = $]]

c) [[$ (5c + 2d)x = 25c + 10d $]]

[[$ x = $]]

d) [[$ (3c + 7d)x = -18c - 42d $]]

[[$ x = $]]