The first evening she received x calls.
The second evening she received 7 fewer calls than the first evening, so she received x-7 calls.
The third evening she received 3 times as many calls as the first evening, so she received 3x calls.
Over the three evenings she received 73 calls altogether.
[tex]x+x-7+3x=73 \\
5x-7=73 \\
5x=73+7 \\
5x=80 \\
x=\frac{80}{5} \\
x=16 \Leftarrow \hbox{the first evening} \\ \\ \hbox{the second evening:} \\
x-7=16-7=9 \\ \\ \hbox{the third evening:} \\
3x=3 \times 16=48[/tex]
The first evening she received 16 calls, the second evening she received 9 calls, the third evening she received 48 calls.