The sum of two numbers is 21. the second number is six times the first number. work out the two numbers.

X + 6x = 21  combine like terms,
7x = 21 simplify
x = 3 this is the smaller number. since both numbers added together equal 21, subtract this number from 21 to get the other number.

21 - 3 = 18

18 is your larger number, 3 is your smaller number, or 
x = 3
y = 18

Hope this helps and have a great day! :)