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

Compute the gradient,
$$ m= \frac{y_2-y_1}{x_2-x_1} $$
Then the y-intercept is
$$ c=y_1 - mx_1 $$
or you could do $c=y_2-mx_2$ which would give the same answer.
Then to find $y$ for a particular $x$ (e.g. $x=200$), you just do
$$y = mx+c$$
