Subtracting a positive number moves you to the left on the number line. Subtracting a negative number moves you in the opposite direction, to the right.
Here, we start at -2 1/2 = -5/2, and we move 1 3/4 = 7/4 to the right from there. Each mark on this number line is 1/4 unit, so we move 7 marks. The results is ...