i want formula for this calculation

[Lenggries]

This is a really trivial algebra question. You have two points (x1,y1) = (2.0, 0.9) and (x2,y2) = (3.75, 0.8), and want to find (x3,y3) = (2.85, ???) (aka you want to solve for y3 given x3 = 2.85). I'm going to assume all three points lie on the same line (If that is not the case, then you have not provided enough information to solve this problem).

The formula for a line is y = ax + b, where b is the y-intercept and a is the slope. Given that you have two points, you can easily solve for both a and b:

a = (y1-y2)/(x1-x2)
b = (x1*y2 - x2*y1)/(x1-x2)

I'll leave it as an exercise for you to actually solve these equations given the two points you have. After that, simply insert the results into the equation of the line to get:
y3 = a * 2.85 + b