How will we understand where the sign should be in the long division for reminder theor

Construct XYZ in which YZ = 4•2 cm angle Y = 120π and XY= 3•9cm draw XP perpendicular to SAY ( produce if required)
If A is an event in A random experiment such that p(A):p(A')=3:5 find p(A) and p(A')
Explain how you got the FORMULA?