When simplifying expressions consider the operation (ie `+ - times divide` ) and, use the rules of algebra to simplify correctly.
In this expression, `d` is known as a "variable." This is because we do not know it's value and therefore it may "vary" according to the expression in which it appears. Letters, and most commonly `x` , are used as variables to signify unknown quantities in algebra and work well in problem solving.
In this expression, the 3 is known as a "constant," because it will always represent a 3 and so it is certainly constant.
We are multiplying the unknown variable, d by 3 a constant and then by another of the same variable, d. The 3, therefore, will not be affected by the multiplication of `3d times d= 3d^2`
The result would have been different if we were adding the ds because 3d + 1d is when the constant 3 is added to the constant 1 to make 4d. Imagine adding 3 apples and 1 apple. You get 4 apples, not 4 apples to the power of 2.
`3d times d = 3d^2`