Thank you for the solution. Will they be able to use this kind of equation in P5?

To be honest, no because algebra is taught in p6. However, some questions such as this where the difficulty is High, is expected to be solved by only a few out of the whole cohort perhaps. So its not unusual to use formulas like this to solve a challenging qns like this. I believe, however, that there may be another way that i am not aware of as there could be more that one way of solving. It is highly advisable and preferablle to consult the qualified teacher in school to give a clearer explanation and working. Hope that answers the question.

Thank you so much, much appreciate your time to help me.

My pleasure! Btw,im a part time tutor offering tuition services for P5 and P6 Math! If you are interested to know more, contact me at 90036443 :)