x = 3 - t ⇒ t = 3 - x
When t = - 3
x = 6 and y = 4 = (6, 4) = initial point
When t = 1
x = 2 and y = 4 = (2, 4) = terminal point
So
y = (t + 1)^2
y = (3 - x + 1)^2
y = (4 - x)^2 = (x - 4)^2 + 0
The vertex is (4, 0)
The motion is right to left { (2,4) is to the left of (6,4) }
See the graph here : https://www.desmos.com/calculator/hxfat2kymd