7th Grade Math Lesson Plans with Sample Problems - Math and Reading Help Example: (-3) + (-5) = -8. A trick for adding numbers that are either both positive or both negative is to add the numbers together and keep the sign they both had. 3) To add a positive number and a negative number, subtract the smaller number from the larger one and keep the sign of the larger number. Examples: (-3) + 5 = 2; 3 + (-5) = -2.