Ted's error is that he did not follow proper order of operations... PEMDAS stands for the order in which you approach all problems in math... Parentheses, exponents, multiplication, division, addition, then subtraction.
Ted multiplied the exponents first instead of completing the operations inside the parentheses. The result was that the sign of his base is incorrect. Here is how it should have gone...
[(-a)^2]^4 =
(a^2)^4 =
a^2×4 =
a^8