To add matrices of the same size, add corresponding entries: top-left with top-left, top-right with top-right, and so on. Matrix addition is done entry by entry. This means each number is added to the number in the same row and same column of the other matrix. Matrix operations require attention to position, so avoid mixing entries from different rows or columns. The correct answer is [[-3, 3], [-3, 9]], which matches the calculation or reasoning shown.