b(7-b)=21

Solution for b(7-b)=21 equation:

Simplifying
b(7 + -1b) = 21
(7 * b + -1b * b) = 21
(7b + -1b2) = 21

Solving
7b + -1b2 = 21

Solving for variable 'b'.

Reorder the terms:
-21 + 7b + -1b2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 7b + -1b2 = 0

Begin completing the square.  Divide all terms by
-1 the coefficient of the squared term: 

Divide each side by '-1'.
21 + -7b + b2 = 0

Move the constant term to the right:

Add '-21' to each side of the equation.
21 + -7b + -21 + b2 = 0 + -21

Reorder the terms:
21 + -21 + -7b + b2 = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -7b + b2 = 0 + -21
-7b + b2 = 0 + -21

Combine like terms: 0 + -21 = -21
-7b + b2 = -21

The b term is -7b.  Take half its coefficient (-3.5).
Square it (12.25) and add it to both sides.

Add '12.25' to each side of the equation.
-7b + 12.25 + b2 = -21 + 12.25

Reorder the terms:
12.25 + -7b + b2 = -21 + 12.25

Combine like terms: -21 + 12.25 = -8.75
12.25 + -7b + b2 = -8.75

Factor a perfect square on the left side:
(b + -3.5)(b + -3.5) = -8.75

Can't calculate square root of the right side.

The solution to this equation could not be determined.