From what I know, 680,705 in expanded form is: 600,000 + 80,000 + 0,000 + 700 + 00 + 5= 680,705 (You might use the zeros, or you might not, I learnt to do it this way!)
You can solve a system of three equations by multiplying each equation by a number that allows you to add or suvtract the same equation together by eliminating the x or y variable
