Final answer:
The constant percent rate of change for the equation y=4.2^x is 320%, which is calculated based on the base of the exponential function.
Step-by-step explanation:
The student asked to complete the statement about the constant percent rate of change for the exponential equation y=4.2x. For any exponential function of the form y=ax, where a is the base and x is the exponent, the constant percent rate of change is derived from the base. In this case, the percent rate of change is (4.2 - 1) × 100% or 320%. This percent represents the rate at which the value of y increases as x increases by 1.