The problem asks for the entries of An where A is a 2x2 matrix. The answers are functions of n, and WeBWorK seems to be accepting some versions of the correct answer but not others.
For instance, if the correct answer is 3·2n + 3n, the student should be able to enter 3*2^n+3^n and get it right. However, WeBWorK says this is wrong.
However, WeBWorK does accept 3*(2)^n+(3)^n as a correct answer. The requirement that the base of the exponent be put in parentheses seems artificial.
Looking over the problem code, I didn't do anything unusual, other than change the tolerance mode to absolute. (This was causing errors when the answer was 0.) However, I noticed that when I was setting up the formulas with the correct answer, I have parentheses there. Is that what's causing the problem?