Find y-coordinate on a line between two (known) points Iam a littlebit stuck with a simple task and hope to find some help here, since my days in school are now long time over and to be honest i can’t remember so well how to do it. 
I have a straight line between two points, lets say (8,20) (300,50) and i want to figure out whats the y-value of (200,y). Now i think i need to find the slope by using (y2-y1)/(x2-x1). But from there i’am stuck. 
Any help is appreciate

If you want to solve this using homogeneous coordinates then you establish the $(A,B,C)$ coefficients of the line forming the equation $Ax+By+C=0$ and then solve for $y$ when $x=200$.
The line coefficients are:
$$ \begin{pmatrix} A \\ B \\ C \end{pmatrix} = \begin{pmatrix} 8 \\ 20 \\ 1 \end{pmatrix} \times \begin{pmatrix} 300 \\ 50 \\ 1 \end{pmatrix} = \begin{pmatrix} -30 \\ 292 \\ -56000 \end{pmatrix}$$ where $\times$ is the vector cross product.
To find your point solve $$ \begin{pmatrix} 200 \\ y \\ 1 \end{pmatrix} \cdot \begin{pmatrix} -30 \\ 292 \\ -56000 \end{pmatrix} =0$$ where $\cdot$ is the vector dot product.
$(200)(-3)+(y)(292)+(1)(-56000)=0 \} y = \frac{2900}{73} $
