Question 536674
(1) {{{ x + y = 12 }}}
(2) {{{ x - y = 4 }}} 
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(2) {{{ x = y + 4 }}}
Substitute this value of {{{x}}}
into equation (1)
(1) {{{ ( y + 4 ) + y = 12 }}}
(1) {{{ 2y = 12 - 4 }}}
(1) {{{ 2y = 8 }}}
(1) {{{ y = 4 }}}
Substitute this back into either (1) or (2)
(2) {{{ x - 4 = 4 }}}
(2) {{{ x = 8 }}}
The solution is (8,4)