We are going to represent the 2 numbers by a and b.
Assume the larger number is a while b is the smaller number.
From the question, the sum of the 2 numbers is 21. Thus:
a+b=21....... eqn 1
The larger number is 6 less than than twice the smaller
Thus:
a=2b-6...... eqn 2
The substitute eqn 2 for a in eqn 1
Thus:
2b-6+b=21.....eqn 3
Collect like terms in eqn 3:
Thus:
2b+b-6=21
3b-6=21
3b=21+6
3b=27
Divide both sides by 3 to find b
3b/3=27/3
b= 9
Substitute b in either of the original 2 equations
Using eqn 1:
a+9=21
Thus:
a=21-9
a=12
Using eqn 2:
a=(2×9)-6
a=18-6
a=12
