To obtain the minimum value of z, the first term of the binomial equation should be as small as possible, while the second term should be as large as possible. Thus, you must choose the lowest boundary of x which is 0. Then, choose the highest boundary of y which is 8. So, we substitute this to the equation:
The absolute value returns the "positive version" of a number. In other words, if is already positive, then . Otherwise, if is negative, then the absolute value changes its sign, so that it returns positive: .
So, in this case, we have to plug the required values for in the expression . Then, if the result is positive we're ok, otherwise we discard the negative sign.