Answer: Square root of 288, or estimated 16.97
Explanation:
Conveniently, the points form a diagonal line as you show, so we can use the Pythagorean Theorem to find the distance.
The x-component of this is the distance between -6 and 6, which is 12, and the y-component is again the distance between -6 and 6, which is also 12.
Therefore, the distance is the square root of 12^2 + 12^2, or the squareroot of 288.