Deduce what number can replace x.

a) [[$ (4^2)^x = 4^6 $]]
[[$ x = $]]

b) [[$ (4^x)^4 = 4^8 $]]
[[$ x = $]]

c) [[$ (5^0)^x = 5^{10} $]]
[[$ x = $]]

d) [[$ (7^x)^x = 7^9 $]]
[[$ x = $]]