Supposedly a matrix $A$ is diagonalisable iff it can be written $A=\sum \lambda_i \pi_i$ where $\lambda_i$ are the distinct eigenvalues of $A$, and $\pi_i$ are projections satisfying $i \neq j \implies \pi_i \pi_j=0$ and $\sum \pi_i=I$. I'm willing to take this fact for granted at the moment.
Given $A=\begin{bmatrix} -6 & 6 \\ -12 & 11 \end{bmatrix}$, I'm not following how my course's notes find the projection matrices $P_1$ and $P_2$ satisfying these properties. It finds the eigenvectors, $\begin{bmatrix} 3 \\ 4 \end{bmatrix}$ and $\begin{bmatrix} 2 \\ 3 \end{bmatrix}$. Then claims that the projection matrices will have their columns being the scalar multiples of these, letting $P_1=\begin{bmatrix}3a & 3b \\ 4a & 4b \end{bmatrix}$ and $P_2=\begin{bmatrix}2c & 2d \\ 3c & 3d \end{bmatrix}$.
Then it uses the aforementioned properties of these projections to form a system of many equations, eventually arriving at $a=3,b=-2,c=-4,d=3$, and $P_1=\begin{bmatrix}9 & -6 \\ 12 & -8 \end{bmatrix}$, $P_2=\begin{bmatrix} -8 & 6 \\ -12 & 9 \end{bmatrix}$.
But why does this process produce a projection matrix? It's not intuitive to me why we start by assuming they have their columns as scalar multiples of the eigenvectors. We've shown that $P_1$ and $P_2$ are matrices satisfying the required properties (or rather defined them to be that way), but a priori I wouldn't know that they're projections.