Hello.
Let the number be u.
Now, let's fine the product of u and -9.
We write it like this:
-9u
 The number (in this case, it's a coefficient) is before the letter (variable)
 The number (in this case, it's a coefficient) is before the letter (variable)
 We can also write it as -9*u, but we usually write it as -9u.
 We can also write it as -9*u, but we usually write it as -9u.
I hope it helps.
Have a nice day.
