To answer this, you basically use Pythagoras' Theroem, but instead of:
![c = \sqrt{a^{2} + b^{2}}](/tpl/images/0055/9298/daed7.png)
it will be :
![distance = \sqrt{(y - y1)^{2} + (x - x1)^{2} }](/tpl/images/0055/9298/9968a.png)
So you are finding the squareroot of the (difference in y coordinates)² plus (difference in x coordinates) ²:
x is the x-coordinate of (0, -1) (so x = 0)
y is the y-coordinte of (0, -1) ( so y = -1)
x1 is the x coordinate of (3, -3) ( so x1 = 3)
y1 is the y coordinate of (3, -3) (so y1 = -3)
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Now, lets find the distance between the two points, by substituting all of this values into the equation at the top:
(substitute in values)
(simplify: note -1 - - 3 = -1 + 3)
(simplify)
(now square the numbers)
(simplify)
![distance = \sqrt{13 }](/tpl/images/0055/9298/b5426.png)
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C. ![\sqrt{13}](/tpl/images/0055/9298/b5ba4.png)