The x shows how they are meeting with the things to be with
If I’m not mistaken I think this is what you’re asking for?
Can you repost with the picture please
T = 5 a
i = 4% aa
j = 1200
c = ...
j = cit/100
c = 100j / it
c = 100*1200 / 4*5
c = 120000 / 20
c = 12000/2
c = 6000
montante
M = c+j
M = 6000+1200
M = 7200
The answer is B.24x^6-18x^5=6x^5(4x-3)