Notice that
(x - 1)²/4 + (y + 2)²/9 = (2 sin(θ))²/4 + (3 cos(θ))²/9
… = sin²(θ) + cos²(θ)
… = 1
Solve for y in terms of x :
(x - 1)²/4 + (y + 2)²/9 = 1
(y + 2)²/9 = 1 - (x - 1)²/4
(y + 2)² = 9 - 9/4 (x - 1)²
y + 2 = ± √(9 - 9/4 (x - 1)²)
y + 2 = ± 3/2 √(4 - (x - 1)²)
y = -2 ± 3/2 √(4 - (x - 1)²)
In order for the square root to be defined, one needs
4 - (x - 1)² ≥ 0
(x - 1)² ≤ 4
-2 ≤ x - 1 ≤ 2
-1 ≤ x ≤ 3
so x must belong to the interval [-1, 3].