Let Brian’s age now be x.

Given “Sam is twice as old as Brian now”:
Sam’s age now = 2x

Given “in w years time, sum of their ages will be 40”:

Brian’s age in w years time = x+w
Sam’s age in w years time = 2x+w
Sum of ages in w years time
= x+w+2x+w
= 3x + 2w

3x + 2w = 40
3x = 40-2w
x= (40-2w)/3
-> question wants final answer to be in terms of w
-> make x the subject so that later when we sub x into the question requirement (aka Brian’s age 5 years ago = x-5) will only have w

Brian’s age 5 years ago
= x-5
= ((40-2w)/3) - 5
=(40-2w-15)/3 -> make numerator the same in order to combine them into single fraction
= (25-2w)/3

Alright! Thanks!