Write the equation of the parabola below

Equation is: y = 0.5x² + 0.5x - 3

Explanation:
general form of the parabola is:
y = ax² + bx + c

Now, we will need to solve for a, b and c.
To do this, we will simply get points from the graph, substitute in the general equation and solve for the missing coefficients.

First point that we will use is (0,-3). 
y = y = ax² + bx + c
-3 = a(0)² + b(0) + c
c = -3

The equation now becomes:
y = ax² + bx - 3

The second point that we will use is (2,0):
y = ax² + bx - 3
0 = a(2)² + b(2) - 3
0 = 4a + 2b -3
4a + 2b = 3
This means that:
2b = 3 - 4a
b = 1.5 - 2a > I

The third point that we will use is (-3,0):
y = ax² + bx - 3
0 = a(-3)² + b(-3) - 3
0 = 9a - 3b - 3
9a - 3b = 3 > II

Substitute with I in II and solve for a as follows:
9a - 3b = 3
9a - 3(1.5 - 2a) = 3
9a - 4.5 + 6a = 3
15a = 7.5
a = 7.5 / 15
a = 0.5

Substitute with the value of a in equation I to get b as follows:
b = 1.5 - 2a 
b = 1.5 - 2(0.5)
b = 0.5

Substitute with a and b in the equation as follows:
y = 0.5x² + 0.5x - 3

Hope this helps :)