Let a and b = the numbers we must find.
a + 2b = 5.....Equation A
2a + b = 13.......Equation B
We have a system of equations in two variables.
I will solve A for little a.
a + 2b = 5
a = 5 - 2b
Now I will plug 5 - 2b for a in B.
2(5 - 2b) + b = 13
10 - 4b + b = 13
-3b = 3
b = -1
To find a, replace b with -1 in EITHER A or B.
I will use Equation A.
a + 2b = 5
a + 2(-1) = 5
a - 2 = 5
a = 7
The two numbers are -1 and 7.
Understand?