Given a budget for two goods $x_1$ and $x_2$, a fixed price for good 2 and three prices for good 1 with the corresponding optimal amount of good 1 ($x_1$), I like to calculate the PED for good 1.
By looking at the three given points, I know that the price consumption curve for good 1 is a horizontal curve, i.e. the slope of the price consumption curve seems to be zero (connecting the three points).
Since I know the price ratio, I also know the MRS for the three given points.
So I have to find $x_1(p_1, p_2, m)$, where $p_2$ and $m$ are known constants, in order to determine $\partial x_1(...)/\partial p_1$.
What is the correct approach to this problem?