The first task is to choose which two endpoints to interpolate between.

1.5 is between 1.4 and 1.6, so it makes sense to fit your line to
those two points.  

You could certainly also fit a line to the x=1.0 and x=1.8 points, or
the x=1.2 and x=1.8 points, etc., but it's generally best to use the
closest points, since they are the best samples that you have of the
function in that range.

Note that to INTERPOLATE, you must pick two points that surround the
value that you're looking for.  

Here's the math:

x1 = 1.4	f(x1) = 4.055
x2 = 1.6	f(x2) = 4.953

Say that the line that fits those two points is f(x) = ax + b.  The task
is now to find a and b.  One way to do that is simply to plug in the
two points:

f(x1) = ax1 + b:	4.055 = 1.4a + b
f(x2) = ax2 + b:	4.953 = 1.6a + b

This is two equations in two unknowns (a and b).  You know lots
of ways to solve this.  The answers are a = 4.49 and b = -2.23

Then, to get f(1.5), you simply plug in x = 1.5:

4.49(1.5) - 2.23 = 4.505

(I wasn't being too careful with sigfigs, so you may get a slightly
different answer.)