Two Numbers with a Sum of 21 and a Difference of 3

Define variables:
Let x = larger number and y = smaller number
Resulting system of equations:
x + y = Sum
x − y = Difference
Solve the system of equations for x (larger number):
x =  

Sum + Difference/2

Find x by substituting given Sum and Difference into equation for x and simplify:
x =  
x =  
x =  12
Solve the system of equations for y (smaller number):
y =  

Sum − Difference/2

Find y by substituting given Sum and Difference into equation for y and simplify:
y =  
y =  
y =  9
Therefore, the two Numbers are 12 and 9
Check Sum of the two Numbers:

	1
	1	2
		9
	2	1

12 + 9 = 21 ✓
Check Difference between the two Numbers:

	0	12
	1	2
−		9
		3

12 − 9 = 3 ✓