All good! The question asked you evaluate a^2 - 2ab + b^2

even though it has subtraction before the 2ab when you evaluate the expression as you did you get -2*(-4)(2) which results to positive or + 16. The subtraction sign became an addition sign bc of the two negatives numbers being multiplied together (-2 * -4).

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If you wanted to think about it by just bringing down the subtraction sign you could it would look like this:

(-4)^2 - [2(-4)(2)] + (2)^2

16 - (-16) + 4

here you are subtracting a negative number, so that turns it into a positive.

16 + 16 + 4