Can you post the code that you have written so far?

Would you like me to post a possible solution or would you like my help in figuring out a solution?

Here is one solution:

let t := this;
let x := first((select AVANCES_PROYECTO)[Semana = t.Semana - 1]);
'Horas Ganadas Real' - x.'Horas Ganadas Real'

It will give you a blank for the first record and it doesn't account for if you have multiple records with the same Semana.