let

x = the age of Ralph now

y = the age of Sara now

Ralph is 3 times as old as Sara

x = 3y

in 6 years, Ralph will be only twice as old as Sara will be then

x + 6 = 2(y + 6)

by solving the system of equations

x = 3y

x + 6 = 2(y + 6)

we find that

x = 18

y = 6

Ralph is 18 years old now.