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.