SOLUTION: Really, expressions involving variables, subtraction: I am trying to solve {{{3y=6x+21 and 3y=-4x+31}}} by subtraction instead of graphing. I have tried 6x+21minus-4x+31 and

Algebra ->  Expressions-with-variables -> SOLUTION: Really, expressions involving variables, subtraction: I am trying to solve {{{3y=6x+21 and 3y=-4x+31}}} by subtraction instead of graphing. I have tried 6x+21minus-4x+31 and      Log On


   

Question 17066: Really, expressions involving variables, subtraction: I am trying to solve 3y=6x%2B21+and+3y=-4x%2B31 by subtraction instead of graphing. I have tried 6x+21minus-4x+31 and came up with 2x + -10 which doesn't work for the problem. How would I find he constant for the variables? What steps do I take to solve it correctly? Thank you for the help!
Answer by rapaljer(4671) About Me  (Show Source):
You can put this solution on YOUR website!
If as you say, 3y=6x%2B21 and 3y=-4x%2B31, then
3y = 3y, so
6x+21 = -4x + 31

It would be a nice idea to solve this equation for x, by adding +4x to each side:
6x + 4x + 21 = -4x + 4x + 31
10x + 21 = 31

Next, subtract 21 from each side:
10x + 21 - 21 = 31 - 21
10x = 10
x= 1

Solve for y next:
3y = 6x + 21
3y = 6(1) + 21
3y = 27
y= 9

Solution (1, 9).

Check: Use the other equation.
3y = -4x + 31
3(9) = -4(1) + 31
27 = 27
It checks!!

R^2 at SCC