The product of 9 and a number is 12 less than three times that number.

what is the number?

So let me give you the answer with procedure so that you can understand it better:
Let x = Number 

Product of 9 and Number:
9x 
is 12 less than 3 times that number. So: 
3x - 12 
Set them equal and solve for x like this:
9x = 3x - 12 
6x = -12 
x = -2
I hope this is very useful for you

1.17+3.99a. you combine the terms with a to create 3.99 and the 1.17 stays because it has no like terms or a variable