Answer
$x^2+y^2=26$.
Work Step by Step
The standard form of the equation of a circle with a center at the origin and radius $r$ is: $x^2+y^2=r^2$.
Plugging in the values of the given point we get: $5^2+(-1)^2=r^2\\26=r^2\\r=\sqrt{26}$
($r$ cannot be negative). Hence our equation: $x^2+y^2=26$.
