"a" and integer is a whole number not a fraction or a decimal
Imagine what x is quantity of stools. So
3x+120=4x
x=120
120 stools in cafeteria
120 chairs in cafeteria
120+120=240
There 240 people can sit in this cafeteria
Formula for mid point.
(x1/2+x2/2 , y1/2+ y2/2)
Mid point=
(1/2+ (-3)/2 , 4/2+ 7/2)
(-2/2 , 11/2)
(-1, 5.5)