Before adding the equations in the elimination method, the coefficients of one variable must be:

1. Adding the equations is meant to make one variable disappear. 2. That only happens if its two coefficients cancel each other. 3. We want the coefficients of one variable to be opposites. 4. Then adding the equations eliminates that variable. _Source: OpenStax Elementary Algebra 2e (CC BY 4.0), Ch 5 'Systems of Linear Equations', sections 5.1-5.3_