Equation of a line through a point (h,k) and with a given slope "m" is given by

(y-k) = m (x-h)

------------------------------

Slope of a tangent at any point h on 4^x is given using differentiation of f(x)..

f(x) = 4^x

f'(x) = 4^x ln(4)

So

Hence the value of m in your question at any point (h,k) " lying on the curve": is

-----------

So equation of tangent is

--------------------

(h,k) is ( 2,16) in this particular problem.

Still in trouble feel free to ask.