Answer:
One number, let's call it x, is 6 more than twice the other number, let's say this number is y.
x= 2y+6
x+y=21
We're left with a system of equations.
If we substitute the first equation in the second we find the value of y.
(2y+6)+y=21
3y+6=21
3y=15
y=5
Now that we have found the value of y, we can take that value and substitute it in the second equation (since it's easier) to find the value of x.
x+ (5)=21
x=16
Now to check if our answers are correct, we plug in both values into any equation and see if they equate.
x+y=21
(16)+(5)=21
21=21
Our solution is correct!