[21-3a]=6-a

Solution for [21-3a]=6-a equation:

Simplifying
[21 + -3a] = 6 + -1a

Remove brackets around [21 + -3a]
21 + -3a = 6 + -1a

Solving
21 + -3a = 6 + -1a

Solving for variable 'a'.

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

Add 'a' to each side of the equation.
21 + -3a + a = 6 + -1a + a

Combine like terms: -3a + a = -2a
21 + -2a = 6 + -1a + a

Combine like terms: -1a + a = 0
21 + -2a = 6 + 0
21 + -2a = 6

Add '-21' to each side of the equation.
21 + -21 + -2a = 6 + -21

Combine like terms: 21 + -21 = 0
0 + -2a = 6 + -21
-2a = 6 + -21

Combine like terms: 6 + -21 = -15
-2a = -15

Divide each side by '-2'.
a = 7.5

Simplifying
a = 7.5