First substitute 'y' in 2nd equation by right side of 1st one and you get:
[tex]Y=6x[/tex]
[tex]6x=5x+7[/tex]
now substract 5x from both sides of 2nd equal and you get:
[tex]6x-5x=5x+7-5x[/tex]
as a result of the equal above you get:
[tex]x=7[/tex]
now substitute x in very first equation by the result from above (x=7):
[tex]Y=6*7=42[/tex]
[tex]x=7[/tex]
