let

m = the age of Mark now

l = the age of Linda now

Mark is 21 years older than his sister Linda:

m = 21 + l

In 6 years, he will be twice as old as Linda:

m + 6 = 2(l + 6)

by solving the system of equations:

m = 21 + l

m + 6 = 2(l + 6)

we find:

m = 36 years

l = 15 years

click here to see the step by step solution of the system of equations:

Mark is 36 years old now.

Linda is 15 years old now.