Explanation:
a reflection across the x-axis simply means that all functional value flip the sign. positive to negative, and negative to positive. in other words, everything that was above the x-axis is now below the x-axis and vice versa. but the distance from the x-axis stays the same (the absolute value of the functional values stays the same).
what mathematical operation does that ?
right, a multiplication by -1.
so,
f(x) = -g(x) = -8×(4^x)