Hello,

rearrange your equation to:

(4^x)^2 - 4*4^x - 21 = 0

That's a quadratic equation in 4^x. So substitute t = 4^x and you'll get:

t^2 - 4t - 21 = 0

Solve this equation for t. (I leave this part for you. There is only one possible solution: 7)

Re-substitute: 7 = 4^x.

Now solve this equation for x = (log(10)7) / (log(10)4) approximately 1.4036...

Greetings

EB