This involves systems of equations. To do this, let's first set up our equation:
x + y = 6
xy = 3
We now can see that x in terms of y is 6-y
Plug that in for x in the second equation, and you get 6y-y^2=3. We can turn this into a quadratic and solve.
-y^2+6y-3=0.
Divide both sides by -1
y^2-6y+3=0
Now, can you solve the quadratic?
Once you have done that, find x, and then you are all set!