
I use substitution when I notice that in one of the equations one of the variables has a coefficient of 1
Then solve for that single variable
e.g. in #2
7x + y = 57 ,
y = 7x + 57
now sub that into the other equation
6x + 4y = 10
6x + 4(7x+57) = 10
6x + 28x + 225 = 10
34x =  218
x = 218/34 =  109/17
then y = 7(109/17) + 57
y = 206/17
do the other two the same way

Okay but the homework is saying the answer is (7,8) so how would they have gotten that from the equation you did above?

I made a transcript error.
After 6x + 4(7x+57) = 10 it should have been
6x + 28x + 228 = 10
I had 2 errors in copying that line, sorry about that
Now i works out


Okay so what about number one it is a + and the a  for the second problem how do you figure a subtitution problem like that? do the signs make a difference?

For number 3 I got (6.7,51.8) is this correct?