Because "y" is + in 2x+y=7, let's subtract 2x from both sides and end up with an expression for y: y = 7 - 2x. Substituting this into 5x - 3y = 23,
5x - 3(7 - 2x) = 23. Multiply out -3(7 - 2x):
5x - 21 + 6x = 23. Combining 5x and 6x, 11x - 21 = 23, or 11x = 44.
Thus, x = 4. Since y = 7 - 2x, subst. x = 4 results in the value of y:
y = 7 - 2(4) = 7 - 8 = -1.
The solution is (4,-1).
