Let a and b be our two numbers.
Here is the set up:
a + b = 15...........A
4a = 2b + 12......B
We have a system of equations in two variables.
Solve equation A for either a or b. I decided to solve for a.
a + b = 15
a = 15 - b
Plug into equation B.
4(15 - b) = 2b + 12
Solve for b.
60 - 4b = 2b + 12
-4b - 2b = 12 - 60
-6b = -12
b = -12/-6
b = 2
We now know that b is 2. To find a, plug b = 2 into either equation A or B.
As you can see, equation A is easier to deal with.
a + b = 15
a + 2 = 15
a = 15 - 2
a = 13
The two missing numbers are 2 and 13.