Drawing a tangent line on plot but keep getting the error "array indices must be positive integers or logical values"?

I have a plot and I picked two points from that plot that I need to use to draw a tangent line.

After choosing the points, I used them to find the slope and then changed it to slope intercept form to find the y value. Then I plotted it.

Here is my code:

%original plot

plot(X1,Y1)

%picking points

x1=3.4959;

y1=1925.72;

x=14.214;

y=8113.27;

%slope formula

k=(y-y1)/(x-x1);

y=k(x-x1)+y1; %this is the line giving me the error

hold on

plot(x,y);

hold off

Why doesn't this work? I was told to compute the slope and then use y-y1=k(x-x1) to draw the line. What did I do wrong?

Answer 1

[Cris LaPierre]

You need to include the multiply operator:

y=k*(x-x1)+y1;

[Astros Aradottir]

That worked, thank you!