Use the Distributive Property:
You "distribute" -6 into the equation by multiplying it by each number in the parentheses.
-6(a+8)
1) multiply -6 by a, which equals -6a
2) multiply -6 by 8, which equals -48
Your equation should be -6a-48
From there, it can be seen that you can divide -6 from each number in the equation.
-6a/-6 just leaves you with a
-48/-6 gives you 8
a-8