Given: (x−α2)+(y−β2)=50
The given equation of circle represent centre as (α,β) and radius as 52 units.
Now, x+y=0 is tangent to the given circle at P.

We know that, radius is perpendicular to tangent at the point of tangency.
⇒CP⊥(x+y=0) and CP=r
⇒r=∣12+12α×1+β×1∣
⇒52=∣2α+β∣
⇒50=2(α+β)2
⇒(α+β)2=100