smc

We see that the center of this circle is at $\left(\frac{-5+25}{2},0\right)=(10,0)$. The radius is $\frac{30}{2}=15$. So the equation of this circle is $$(x-10{)}^2+y^2=225.$$ Substituting $y=15$ yields $(x-10{)}^2=0$ so $x=\boxed{10}$.