Solving Linear Equation in two variable problem

I don't know if there is better solution. This is the way I solved it.

Let $x$ and $y$ be the numbers. Then,

$(x-y)^2=x+y$

Now,use the given options (elimination method). Let $x=28$ and $y=21$ .

$(x-y)^2=x+y$

$(28-21)^2=28+21$

$7^2=49$

$49=49$

So the numbers are $\boxed{28}$ and $\boxed{21}$ .