Question 450673
Lets create equations from what we know:

Equation 1) the sum is 39 so:  a + b = 39
Equation 2) one number is three more than half the other..so: a/2 + 3 = b

I would get rid of the fraction in equation 2 by multipling the equation by 2
2(a/2 + 3 = b)
a + 6 = 2b  then I would set it up to show a single variable to equal the other
SO:
a + 6 = 2b subtract 6 from both sides
a = 2b - 6  now we can use this equation to substitute into equation 1

2b - 6 + b = 39
3b - 6 = 39  add 6 to both sides
3b = 45  divide both sides by 3
b = 15
Now that you know b, plug that into one of the equations and solve

a + 15 = 39  substract 15 from both sides
a = 24

So now you know that the two numbers are 15 and 24