Given that $2^x = 7$, find the value of $4^{x-1}$.

2^x  = 7

4^(x  -1) =    4^x / 4  =  (2^2)^x / 4 = (2^x)^2  /4  = (7)^2/ 4  =  49/4

omg thx so much cphil