could someone help me understand this?

Question

4x-3y=5

x=2y+5

 

I've tried so many ways to do it correctly and this is what i came up with...

 

4x-3(x+5)=5

4x-3x-15=5

and i've tried adding 15 to both sides so it would be x=20 but that is incorrect. What do i do? 

 

please help im so frustrated 

Answer 1

Dear Jared

 

You are correct about the procedure to resolve this problem. But, you are doing a mistake when you substitute variables.

 

Let me explain it:

 

   Your Procedure               My Procedure

 

     4x-3(x+5)=5                4(2y + 5) - 3y = 5    (I am replacing x by 2y + 5)
     4x-3x-15=5                  8y - 3y +20 = 5

                                        5y = 5 - 20

                                        5y = -15

                                          y = -3

 

                                        But: x = 3y + 5     Replacing y by its value (-3)

                                               x = 2(-3) + 5

                                               x = -6 + 5

                                               x = -1

 

                                       Proof:   4x-3y=5    If x=-1 and y=-3  then    4(-1) - 3(-3) = -4 + 9 = 5    5 = 5

                                                   x=2y+5    If x=-1 and y=-3  then     -1 = 2(-3) + 5   -1 = -6 + 5   -1 = -1