STEP 1: Recall what are exponential functions

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STEP 2: Calculate the difference in the x- and the y-values

As the independent variable x changes by a constant amount, the dependent variable y is multiplied by a constant factor, which means consecutive y-values form a constant ratio.

As the x values increase by 1 the y values are multiplied by 2. So, this is an exponential function.

Skills you may further want to recall:

What is a variable

https://www.mathsisfun.com/definitions/variable.html

What is a dependent and an independent variable

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What is a constant

https://www.mathsisfun.com/definitions/constant.html

What are consecutive numbers