Algebra 1: Common Core (15th Edition)

$\sqrt 900$ means that we are supposed to find what number times itself equals 900. We recall that 30 times 30 is 900, so $\sqrt 900$ is 30. However, as we can see, there is a negative outside of the square root. This means that we multiply $\sqrt 900$, 30, by -1, giving us our final answer: -30.