x+y=670

7x+5y=3740
__________
-7x-7y=4690
-2y=-900
y=450

i don't get this at all, can somebody please explain? thanks(:

They multiplied the first equation, which was x+u = 670 by -7 to get

-7x - 7y = -4690 (you had 4690, which is wrong)
then adding that to the second equation of
7x + 5y = 3740 gives

-2y = - 950
y = 475
now put that back into x+y = 670
x+475 = 670
x = 195

check in the second equation
7(195) + 5(475)
= 3740 = right side

checks!