What do you mean by find the tangent?

I'll assume you mean gradient and start with finding the gradient of the tangent.

The gradient of the tangent can be found with , where y2 is the biggest y-coordinate you have ( which is 3 in your case ) and y1 is the smallest y coordinate you have. Same thing applies for X2 and X1. So sub all values in, you should end up with and therefore the gradient of the tangent is 1.