function to sum a column's entries based on another column

Hello. Thanks for reading. I am grateful for your expertise.

I'm an intermediate excel user, and comfortable with nested if statements and other functions, but there's one task I repeatedly come across that I don't know how to do efficiently.

Let's say I have a dataset with blank or X in column A, and a dollar value in column B. I want to sum the dollar amounts for column B only if there is a corresponding X in that row in column A. The way I currently do that is by creating a new column C, making a if statement to display the value of B only if there is an entry in A, and then doing operations on that new range. I figure there has to be an easier way. I don't know how to use VLOOKUP, despite trying to read the helpme a bunch of times. Is that relevant?