Complete the square: x^2-8x+21=6

Please show work

What is the approximate total length of iron edging needed to create the square frame and the two diagonals?

According to the number line, what is the distance between points a and b? v ++ + + + + -16 -14 -12 -10 -8 + 6 + 4 + -2 + 0 + 2 + 4 + 6 + 8 + 10 + 12 + 14 + 16 o6 units o 7 units 12 units o 14 units

How do i do this activity, is appreciated