Originally Posted by

**Aryth** Let's say that at t = 0, it is the year 1995, so we know that:

$\displaystyle E(0) = 69.5$

So, at t = 4, we get:

$\displaystyle E(4) = 72.1$

We know that:

$\displaystyle E(x) = at + b$

Where b is the y-intercept, which is $\displaystyle E(0)$, that gives us:

$\displaystyle E(t) = at + 69.5$

Now, all we need to to is set it equal to $\displaystyle E(4)$ to solve for a:

$\displaystyle E(4) = a(4) + 69.5 = 72.1$

$\displaystyle 4a = 2.6$

$\displaystyle a = 0.65$

So, we now have an equation:

$\displaystyle E(t) = 0.65t + 69.5$

And, now we find t = 10:

$\displaystyle E(10) = 0.65(10) + 69.5 = 6.5 + 69.5 = 76$

And there you go.