Let George's age be x and Janet's age be y

At present:

Given, George's age = Janet's age + 7

x = y + 3

x - y = 3 .....(1)

After 7 years, George's age will be x + 7 and that of Janet will be y + 7

Given,  Janet's age = 6/7 of George's age

y + 7 = (6/7) x ( x + 7)

7y+ 49 = 6x + 42

6x - 7y = 7....(2)

Multipling equation (1) with 7 we get

7x - 7y = 21....(3)

Subtracting equation (2) from equation (3)

x = 14

From equation (1) 

y = 11

So, george is 14 years old and Janet is 11.