-21=3(1+8a)

Solution for -21=3(1+8a) equation:

Simplifying
-21 = 3(1 + 8a)
-21 = (1 * 3 + 8a * 3)
-21 = (3 + 24a)

Solving
-21 = 3 + 24a

Solving for variable 'a'.

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

Add '-24a' to each side of the equation.
-21 + -24a = 3 + 24a + -24a

Combine like terms: 24a + -24a = 0
-21 + -24a = 3 + 0
-21 + -24a = 3

Add '21' to each side of the equation.
-21 + 21 + -24a = 3 + 21

Combine like terms: -21 + 21 = 0
0 + -24a = 3 + 21
-24a = 3 + 21

Combine like terms: 3 + 21 = 24
-24a = 24

Divide each side by '-24'.
a = -1

Simplifying
a = -1