Simultaneous+Equations

Not sure how to use the method of SUBSTITUTION.  media type="youtube" key="8ockWpx2KKI" height="344" width="425"

Not sure how to use the method of ELIMINATION media type="youtube" key="xB-oXaCoJoc" height="344" width="425"

media type="youtube" key="8ockWpx2KKI" height="344" width="425"