Do I have to use Customs formulas often and when?

Hi Anita, Thank you for your post and welcome to the forum, I hope you enjoyed your course, in answer to your question; It very much depends on the way that you have to use Excel, using formulas is necessary if you want to manipulate the values you have in your Worksheet, if you are going to be writing your own formulas (ie custom formulas) you should consider the acronym BODMAS when you create your formula, BODMAS = Brackets, Ordinals, Division, Multiplication, Addition, Subtraction - this is the order in which Excel works out a formula's operators; Parentheses aka brackets.
Using brackets in your formulas is a good habit to get into even when they are not strictly needed because they help to clarify the order and make the formula easier to understand. It is also possible to put parentheses within parentheses. When parentheses are nested the ones on the inside are evaluated first.
When you enter parentheses you will notice that Excel tries to help you by making the matching parentheses bold as you pass over it with the mouse. Your parentheses must balance. For every open parentheses there must be a corresponding close parentheses.
If you try to submit a formula which does not contain the same number of open and close parentheses, Excel will prompt you and tell you that the formula you have entered is incorrect.
I hope that has made it a little clearer, if so, please click the resolved link, best regards Pete.