Answer:
Step-by-step explanation:
[tex]\left \{ {{x-2y=15} \atop {6x+3y-42=0}} \right. \\[/tex]
[tex]\left \{ {{x=15+ 2y } \atop {6(15+ 2y )\\+3y-42=0}} \right. \\[/tex]
[tex]\left \{ {{x=15+ 2y } \atop {90+ 12y\\+3y-42=0}} \right. \\[/tex]
[tex]\left \{ {{x=15+ 2y } \atop {15y\\+48=0}} \right. \\[/tex]
[tex]\left \{ {{x=15+ 2y } \atop {y\\=-16/5}} \right. \\[/tex]
[tex]\left \{ {{x=43/5\\} \atop {y\\=-16/5}} \right. \\[/tex]
=> 3* 43/5 + 16/5 + z = 15
=> z = -14
