2x + y = 7
5x + y = 9
solve the system of equations.

The first step of solving a system of equations is to solve one equation for a variable. I'll solve the first one for y.

2x + y = 7                     Subtract 2x from both sides
        y = -2x + 7

Now, plug that y value into the y of the second equation and solve for x.

            5x + y = 9          Substitute
5x + (-2x + 7) = 9          Adjust the signs to get rid of parentheses
    5x - 2x + 7 = 9          Combine like terms
           3x + 7 = 9          Subtract 7 from both sides
                 3x = 2          Divide both sides by 3
                   x = 

Now, plug that into the first equation.

2x + y = 7     Substitute
2() + y = 7      Multiply
+ y = 7     Subtract  from each side
y = 5 

The answer to the system of equations is (, 5 ).