Question: I am having trouble calculating the quadratic curve $f(x)$ that goes through 3 points;
for example a curve that goes through $A(1,3), B(-1,-5), and C(-2,12)$.
I can only guess that the curve is upwards and that I may create the system:
$$
y\_1 = ax^2\_1 + bx\_1 + c\\
y\_2 = ax^2\_2 + bx\_2 + c\\
y\_3 = ax^2\_3 + bx\_3 + c
$$
assuming that the points are in the format $A(x,y)$ and from this point what do I do?
Do I build a matrix and use the Gaussian eliminations?

EDIT: I also know that $f(4) = 120$

Answer: 
Each of the points (1,3), (-1,-5) and (-2,12) satisfies the equation $y = ax^2 + bx + c$ for some unknown a,b,c. The task is to find a,b and c. Start by substituting each of the points into the equation, we have

$$
\begin{align}
3 &= a(1)^2 + b(1) + c \\
-5 &= a(-1)^2 + b(-1) + c \\
12 &= a(-2)^2 + b(-2) + c
\end{align}$$
 We can write this more compactly as a matrix equation
$$
\begin{bmatrix} 1 & 1 & 1\\ 1 & -1 & 1\\ 4 & -2 & 1 
\end{bmatrix} \begin{bmatrix}a\\ b\\ c \end{bmatrix} = \begin{bmatrix} 3\\ -5\\ 12\end{bmatrix}
$$

Write the augmented matrix and do elementary row operations 
$$
\begin{bmatrix} 1 & 1 & 1 & 3\\ 1 & -1 & 1& -5\\ 4 & -2 & 1 & 12
\end{bmatrix} \Rightarrow \begin{bmatrix} 1 & 1 & 1 & 3\\ 0 & -2 & 0 & -8\\ 0 & -6 & -3 & 0
\end{bmatrix} \Rightarrow \begin{bmatrix} 1 & 1 & 1 & 3\\ 0 & -2 & 0 & -8\\ 0 & 0 & -3 & 24
\end{bmatrix} 
$$
and now, back substitution.
starting with the last row, 
$$\begin{align}-3c &= 24 \\ c &= -8 \end{align} $$
and then the second row
$$\begin{align} -2b &= -8 \\ b &= 4 \end{align}$$
and finally back substituting these into the first row
$$\begin{align}a + b + c &= 3 \\ a + (4) + (-8) &= 3 \\ a &= 7\end{align}$$

So, I think the equation is:
$$ y = 7x^2 + 4x -8 $$

Please check my work, I did this in a hurry.
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$\ds{A\pars{1,3},\ B\pars{-1,-5},\ C\pars{-2,12}}$
$$
{\rm y}\pars{x} \equiv A\pars{x + 1}\pars{x + 2} + B\pars{x - 1}\pars{x + 2} + C\pars{x - 1}\pars{x + 1}\tag{1} 
$$

> 
> $$
> \begin{array}{rcrcrcrcrcl}
> {\rm y}\pars{1} & = & 3 & \imp & 6A & = & 3 & \imp & A & = & \half
> \\[3mm]
> {\rm y}\pars{-1} & = & -5 & \imp & -2B & = & -5 & \imp & B & = & {5\over 2} 
> \\[3mm]
> {\rm y}\pars{-2} & = & 12 & \imp & 3C & = & 12 & \imp & C & = & 4 
> \end{array}
> $$
>  Reduce , if desired, expression $\pars{1}$. 
> 
> 
>