21=3(2-a)

Solution for 21=3(2-a) equation:

Simplifying
21 = 3(2 + -1a)
21 = (2 * 3 + -1a * 3)
21 = (6 + -3a)

Solving
21 = 6 + -3a

Solving for variable 'a'.

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

Add '3a' to each side of the equation.
21 + 3a = 6 + -3a + 3a

Combine like terms: -3a + 3a = 0
21 + 3a = 6 + 0
21 + 3a = 6

Add '-21' to each side of the equation.
21 + -21 + 3a = 6 + -21

Combine like terms: 21 + -21 = 0
0 + 3a = 6 + -21
3a = 6 + -21

Combine like terms: 6 + -21 = -15
3a = -15

Divide each side by '3'.
a = -5

Simplifying
a = -5