algebra problem

We write the equation in 3x-7y=21 first. 

3x-7y=21

7y=3x-21

y=3/7x-3

To find the x-intercept, we plug in the value 0 for y.

0=3/7x-3

3/7x=3

x=7

To find the y-intercept, we plug in the value 0 for x.

y=3/7(0)-3

y= -3

The x-intercept is (7,0) and the y-intercept (0, -3).

To find the distance, we plug the points into the distance formula.

$\sqrt{(7-0)^2+0-(-3)^2}\\ \sqrt{49+7}\\ \sqrt{58}\\ $

Therefore the distance between the x and y intercept is $\boxed{\sqrt{58}}$.