Hi and welcome to the forum

Actually, its more than just corss your i's and dot your t's lol, its about making the title something that will come up in a search for something similar. "So simple, yet I cant...." is hardly the wording that anyone would use in a search

OK having said that, and if I understand you correctly, it sounds like you need to look at using the sumproduct() function - that is, if you want the total of A1*X1 + A2*X2 + A3*X3 etc.

maybe something like =sumproduct(A1:A10*B1:B10)