Over the last three evenings, latoya received a total of 79 phone calls at the call center. the first evening, she received 6 fewer calls than the third evening. the second evening, she received 3 times as many calls as the third evening. how many phone calls did she receive each evening?

We need to represent the numbers of calls symbolically:

Third evening:      x calls
First eve:              x-6 calls
Second evening:  3x calls

Find x.  Add up these three numbers    x, x-6 and 3x and equate your sum to 79:

x+x-6+3x = 79.  Then 5x=79+6, or 5x = 85.  Thus, x = 17.

17 calls on the 3rd night, 
17-6 calls on the 1st night, and 
3(17) = 51 calls on the 2nd night.

Check!  Add together 17, 11 and 51.  Do these add up to 79?   Yes.