Where y2 is the y value of the second point, y1 is the y value of the first point, x2 is the value of the second x value, and x1 is the x value of the first point.
4 legged tables : x
3 legged stools : y
x + y = 20 y= 20 - x
4x + 3y = 68
Replace y by 20 - x so we only have one letter
4x + 3(20 - x) =68
4x + 60 - 3x =68
x + 60 = 68
x = 8
To find y replace x by 8
y = 20 - 8
y = 12
So you have 8 four legged chairs and 12 three legged stools.
Hope I helped!