Answer: Choice C. [tex]3\sqrt{17}[/tex]
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Work Shown:
[tex]d = \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}\\\\d = \sqrt{(12-15)^2 + (-21-(-9))^2}\\\\d = \sqrt{(12-15)^2 + (-21+9)^2}\\\\d = \sqrt{(-3)^2 + (-12)^2}\\\\d = \sqrt{9 + 144}\\\\d = \sqrt{153}\\\\d = \sqrt{9*17}\\\\d = \sqrt{9}*\sqrt{17}\\\\d = 3\sqrt{17}\\\\d \approx 12.3693169\\\\[/tex]
I used the distance formula for those steps above.
The two points were (x1,y1) = (12,-21) and (x2,y2) = (15,-9) but you could swap the order.
As an alternative route, you can plot the two points on the same xy grid. Then form a right triangle such that the hypotenuse is between the two points mentioned. The pythagorean theorem should lead you to the same answer as above. In fact, the distance formula is basically a modified version of the pythagorean theorem.
