#### Answer

The distance between the first point and the fourth point is $3\sqrt{10}$ units.

#### Work Step by Step

RECALL:
The distance $d$ between the points $(x_1, y_1)$ and $(x_2, y_2)$ is given by the formula
$d=\sqrt{(x_1-x_2)^2+(y_1-y_2)^2}$
Use the formula above to obtain:
$d=\sqrt{(0-3)^2+(-6-3)^2}
\\d=\sqrt{(-3)^2+(-9)^2}
\\d=\sqrt{9+81}
\\d=\sqrt{90}
\\d=\sqrt{9(10)}
\\d=3\sqrt{10}$