Instead of doing yours for you, I'll do one exactly like it so you can
use it as a model to do yours by.  The one I'll do for you is:

Let x be one of the nonnegative numbers.
Then, since their sum is 10, the other nonnegative number is 10-x

Let y = their product = x(10-x) = 10x-x2 = -x2+10x

We graph y = -x2+10x where a=-1, b=10, c=0


 
The vertex is the highest point. So we find the coordinates of the vertex,
using the vertex formula:

1.  x-coordinate of vertex = 

2.  y-coordinate, substitute x-coordinate in equation for parabola,


So the vertex is (5,25) which occurs when x=5

So one of the nonnegative numbers is x = 5
The other nonnegative number is 10-x = 10-5 = 5

So the product of two nonnegative numbers whose sum is 10 is a maximum 
of 25 when the two nonnegative numbers are both 5.

Now do yours the same way.

Edwin