So the equation is ab+cd. So you have to substituting the numbers they gave you into the letters. Therefore it'd be 2 (which is a) times -3 (which is b) plus 4 (which is c) times -5 (which is d). Now you have to multiply each side, so 2 times -3 is -6. Then you multiply the other side, so 4 times -5 is -20. So you add -6 and -20, and you get -26