Decide whether the given equation is an identity, a conditional equation, or a contradiction. Give the solution set. \[ 0.5(x+3)-0.8(x+3)=-0.3 x-0.9 \]

Step 1 ：The given equation is a linear equation in one variable. We can solve it by simplifying both sides of the equation and then isolating the variable on one side. If the equation simplifies to a true statement, such as \(0=0\), it is an identity. If it simplifies to a false statement, such as \(0=1\), it is a contradiction. If we get a specific value for the variable, it is a conditional equation.

Step 2 ：\(x = x\)

Step 3 ：\(-0.3*x - 0.9 = -0.3*x - 0.9\)

Step 4 ：The solution set is \({0.0}\), which means the equation is a conditional equation. The equation is true only when \(x = 0.0\).

Step 5 ：Final Answer: The given equation is a \(\boxed{\text{conditional equation}}\) with the solution set \(\boxed{\{0.0\}}\).