in step 5, you are left with a positive or a negative result. This means that your answer is wrong and you should have checked it before posting this.

see step 7 or video.... <br>same focus mathematical !!!! <br> <br>

In this case, if the 4 part of the equation will be negative, so the answer will be 0.5 = -4 - -4.5 in order to satisfy the right side of the equation. This convention applies when the 5 part of the equation is negative -5 - -4.5 = -0.5. This means that the 4 part will equal -0.5 and is satisfied. When you square root, there has to be a negative and positive answer. You have to factor out -1 from the equation to eliminate ambiguity.

I hate to throw a wrench into your instructable, but in the step giving values to "a" and "b"; "a" was given two different values in the same equation 4 on one side and 5 on the other. Kudos on the attempt, but but a few more "confounding" intermediate steps will hide the "trick" better.

It is mathematical trick, focus...