I assume the 2 after the j is a square. So, we bring the 21 to the other side by subtracting it from both sides, getting 144j^2=64. Now we divide to get 64/144 = j^2. Make sure NOT to simplify it, it's perfect right now. The square root of that is , or 8/12, which we can now simplify to 2/3. Hope this helps!
