let

x = the present age of Nelson

y = the present age of Maria

Nelson is twice as old as Maria

x = 2y

6 years ago, Nelson was five Times as old as Maria

x - 6 = 5*(y - 6)

by solving the system of equations

x = 2y

x - 6 = 5*(y - 6)

we find

x = 16 years

y = 8 years

6 years ago Nelson was 16 - 6 = 10 years old.