Question

How to write as equation, the product of 7 and a number

Answer 1

Let us say that "a number" is x. If you were to do this, then the product of 7 and a number, x would be 7x.

Answer 2

You would consider it as "7n=x" as n is the undefined number and since it is an equation, it must have an equal sign, so x is the product.