[tex]\sf \sqrt{15^2+1+215=x+15}\\\\\\\sf x+15=\sqrt{15^2+1+215}\\\\\\\sf x+15=\sqrt{15^2+216}\\\\\\\sf x+15=\sqrt{225+216}\\\\\\\sf x+15=\sqrt{441}\\\\\\\sf x+15=\sqrt{21^2}\\\\\\\sf x+15=21\\\\\\\sf x+15-15=21-15\\\\\\\to \boxed{\boxed{\tt x=6}}[/tex]
