2(2.5+8.5a)=-28.5

Solution for 2(2.5+8.5a)=-28.5 equation:

Simplifying
2(2.5 + 8.5a) = -28.5
(2.5 * 2 + 8.5a * 2) = -28.5
(5 + 17a) = -28.5

Solving
5 + 17a = -28.5

Solving for variable 'a'.

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

Add '-5' to each side of the equation.
5 + -5 + 17a = -28.5 + -5

Combine like terms: 5 + -5 = 0
0 + 17a = -28.5 + -5
17a = -28.5 + -5

Combine like terms: -28.5 + -5 = -33.5
17a = -33.5

Divide each side by '17'.
a = -1.970588235

Simplifying
a = -1.970588235