Which equation represents an exponential function?

3
x
4
3
x
4
3 x to the 4th power

3
(
4
)
x
3
(
4
)
x
3 times 4 to the x th power

3
x
−
4
3
x
−
4
3 x to the negative 4 power

3
x
3
x
3 x