21=0.5*3(b+5)

Solution for 21=0.5*3(b+5) equation:

Simplifying
21 = 0.5 * 3(b + 5)

Reorder the terms:
21 = 0.5 * 3(5 + b)

Multiply 0.5 * 3
21 = 1.5(5 + b)
21 = (5 * 1.5 + b * 1.5)
21 = (7.5 + 1.5b)

Solving
21 = 7.5 + 1.5b

Solving for variable 'b'.

Move all terms containing b to the left, all other terms to the right.

Add '-1.5b' to each side of the equation.
21 + -1.5b = 7.5 + 1.5b + -1.5b

Combine like terms: 1.5b + -1.5b = 0.0
21 + -1.5b = 7.5 + 0.0
21 + -1.5b = 7.5

Add '-21' to each side of the equation.
21 + -21 + -1.5b = 7.5 + -21

Combine like terms: 21 + -21 = 0
0 + -1.5b = 7.5 + -21
-1.5b = 7.5 + -21

Combine like terms: 7.5 + -21 = -13.5
-1.5b = -13.5

Divide each side by '-1.5'.
b = 9

Simplifying
b = 9