Which equation does not belong with the other three?

Responses

y=3^x
y is equal to 3 to the x th power

f\left(x\right)=2\left(4\right)^x
f of x is equal to 2 times 4 to the x the power

f\left(x\right)=\left(-3\right)^x
f of x is equal to negative 3 to the x the power

y=5\left(3\right)^x
y is equal to 5 times 3 to the x the power
Question 2
Why did you select this equation?

Because it is the only equation that ___ an exponential function.