Distances in 2- and 3-dimensions (and even higher dimensions) can be found using the Pythagorean theorem. The straight-line distance can be considered to be the hypotenuse of a right triangle whose sides are the horizontal and vertical differences between the coordinates.
Here, you have A = (0, 0) and B = (3, 6). The horizontal distance between the points is ...
... 3 - 0 = 3 . . . . the difference of x-coordinates
The vertical distance between the points is ...
... 6 - 0 = 6 . . . . the difference of y-coordinates
Then the straight-line distance (d) between the points is found from the Pythagorean theorem, which tells you ...
... d² = 3² + 6²
... d = √(9 + 36) = √45 ≈ 6.7 . . . units