Question:

If the standard deviation of the numbers 2, 3, a, and 11 is 3.5, then which of the following is true?

3a²−6a+55=0

3a²−8a+84=0

3a²−10a+91=0

3a²−3a+44=0

Variance=Σx²/n−(Σx/n)²
→(3.5)²=(4+9+a²+121)/4−(16+a/4)²
→3a²−8a+280=16(72)²=4×49
→3a²−8a+84=0