I am looking to calculate the changed in mental health scores of each individual between two timepoints.

Each user has a name, and a mental health score from 3 different timepoints. I would like to calculate the change in mental health score between timepoint 3 and 1

Below is example of df I'm starting with:

```
User   Timepoint   Mental Health Score
Bill       1               5
Bill       2               10
Bill       3               15
Wiz        1               10
Wiz        2               10
Wiz        3               15
Sam        1               5
Sam        2               5
Sam        3               5

```

This is desired output:

```
User   Timepoint   Mental Health Score      Change in Mental Health (TP1 and 3) 
Bill       1               5                               
Bill       2               10
Bill       3               15                              10
Wiz        1               10
Wiz        2               10
Wiz        3               15                              5
Sam        1               5
Sam        2               5
Sam        3               5                               0

```

Does anyone know how to do this?