It's impossible to say what the third step is in an acceleration problem without knowing the specific problem and the steps you're already taking.

However, here are some general steps involved in acceleration problems:

1. Identify the known quantities: This might include initial velocity (v_i), final velocity (v_f), time (t), or displacement (d).

2. Determine the unknown quantity: What are you trying to solve for? Is it acceleration (a)? Final velocity? Time?

3. Choose the appropriate equation: There are several equations of motion that can be used depending on the knowns and unknowns. Some common ones are:

* v_f = v_i + at (when acceleration is constant)

* d = v_it + ½at² (when acceleration is constant)

* v_f² = v_i² + 2ad (when acceleration is constant)

4. Plug in the values: Substitute the known quantities into the chosen equation.

5. Solve for the unknown: Perform the necessary calculations to find the answer.

Example:

Let's say you need to find the acceleration of a car that starts from rest (v_i = 0 m/s), reaches a final velocity of 20 m/s in 5 seconds.

* Step 1: Known quantities: v_i = 0 m/s, v_f = 20 m/s, t = 5 s.

* Step 2: Unknown quantity: a (acceleration).

* Step 3: Equation: v_f = v_i + at

* Step 4: Plug in values: 20 m/s = 0 m/s + a * 5 s

* Step 5: Solve for a: a = 20 m/s / 5 s = 4 m/s²

Therefore, the third step in this specific problem is to choose the appropriate equation (v_f = v_i + at).

Please provide the specific problem you're working on, and I can help you identify the third step more accurately.