 Solve one-step addition and subtraction equations by using a symbol to represent an unknown number ... by explaining what the symbol in a given addition or subtraction equation stands for. In the equation 3 + ▲ = 10, the triangle stands for the number (7) that would make the equation true. by creating an addition or subtraction equation with a symbol to represent the unknown operation. In the equation 3 ▲ 7 = 10, the triangle stands for the number (7) that would make the equation true.  by giving a different symbol for the unknown in a given addition or subtraction equation.  by solving, using manipulatives, a given addition or subtraction equation with a symbol to represent the unknown operation. by solving a given addition or subtraction equation with one unknown, using strategies, such as guess and test. by solving a given addition or subtraction equation when the unknown is on the left or the right side of the equation.  by explaining why the unknown in a given addition or subtraction equation can only have one possible value.