There are 5 years (odd), so code time about the middle year 2015 with $X = -2,-1,0,1,2$ so that $\sum X = 0$. …

There are $n = 5$ years (odd), so we take the middle year 2015 as origin and let $X$ be the deviation in years: $X = -2, -1, 0, 1, 2$. Then $\sum X = 0$ and the normal equations reduce to $a = \dfrac{\sum Y}{n}$ and $b = \dfrac{\sum XY}{\sum X^2}$.

Hence

The trend line is $Y_t = 20 + 3.5X$ (origin 2015, unit of $X$ = 1 year).

Trend values (substitute each $X$):

One may instead solve the full normal equations $\sum Y = na + b\sum X$ and $\sum XY = a\sum X + b\sum X^2$ without coding, but coding time so …

Forgetting that the $X$-unit is one year, then mis-reading $b$. Here $b = 3.5$ means sales rise by Rs 3.5 lakh per year. Also, a useful …