M = Mark´s present age
B = Brother´s present age
Mark is 10 years older than his brother mean:
M = B + 10
After four years Mark will be M + 4 yrs old and brother wil bee B + 4 yrs old.
In four years he will be twice as old as his brother mean:
( M + 4 ) / ( B + 4 ) = 2
Now you must solve system:
M = B + 10
( M + 4 ) / ( B + 4 ) = 2
Try that.
The solution is:
M = 16 , B = 6