You are not dumb. No one is dumb. Einstein failed math in high school.
Anna is x years old.
Jane is y years younger.
This means the Jane is x - y years old.
In three years, Anna will be x + 3 years old.
In three years, Jane will be (x - y) + 3 years old.
Let S = the sum of their ages in 3 years.
S = x + 3 + (x - y) + 3
S = 2x + 6 - y
Got it?