Hi Malavika,
We are using the distance formula to find the distance between 2 points. If two points given are (a,b) and (c,d), then the distance between the points is given as,
$$d\ =\ \sqrt{\ \left(a\ -\ c\right)^2\ +\ \left(b\ \ -\ d\right)^2}$$
Here the base points are given as (a, a) and (a + 2, a). So the distance d becomes
$$d\ =\ \sqrt{\left(a\ +\ 2\ -a\right)^2\ +\ \left(a\ -\ a\right)^2\ }\ \ =\ \ \sqrt{\ 2^2\ +\ 0}\ =\ \sqrt{\ 4}\ =\ 2$$.
This is how we calculated the length of the base as 2.
hope this clears your doubt!