ANSWER
[tex]f(x) = 4( {2})^{x} [/tex]
EXPLANATION
Given the exponential function,
[tex]y = f(x)[/tex]
Then the function
[tex]y =a f(x)[/tex]
is stretched vertically by a factor of 'a' units, for a>0.
Based on this, if a=4 and
[tex]f(x) = {2}^{x} [/tex]
Then the equation of the new function is
[tex]f(x) = 4( {2})^{x} [/tex]
