Question 1204081
the combined age of 2 brothers is 35 years 

let the brother's ages be x & y

x+y=35.................................................(1)

7 years ago one brother was twice the age of the other. 

7 years ago
One brother was x-7 years
The other was y-7 years

x-7 = 2(y-7)
x-7 =2y-14
x-2y = -7............................................(2)

subtract (2) from (1)

x+y=35
x-2y = -7

3y =42
y = 14

x+y =35
x+14=35

x= 21

Since we know their present ages we can find how many years apart were they born