Question 117448
From your graph, you have 9 (x,y) data pairs.
First, make a table with the x,y data points. 
Add 5 columns to your table. 
1.{{{x[i]}}}
2.{{{y[i]}}}
3.{{{(x[i]-x[a])}}}
4.{{{(y[i]-y[a])}}}
5.{{{(x[i]-x[a])^2}}}
6.{{{(y[i]-y[a])^2}}}
7.{{{(x[i]-x[a])(y[i]-y[a])}}}
where {{{x[a]}}} is the average of the x values and {{{y[a]}}} is the vaverage of the y values.
Calculate the values for i=1 to 9.
Sum columns 5,6,7.
You are solving for the regression equation 
{{{y=b[1]x+b[0]}}}
where 
{{{b[1]=sum((x[i]-x[a])(y[i]-y[a]))/sum((x[i]-x[a])^2)}}}
or
{{{b[1]=(Sum_of_column_7)/(Sum_of_column_5)}}}
and 
{{{b[0]=y[a]-b[1]*x[a]}}}
From this we see that
{{{b[1]}}} is the slope of the regression line.
{{{b[0]}}} is the y-intercept of the regression line.
{{{y=b[1]x+b[0]}}} is the equation of the regression line.
To calculate the {{{r^2}}}, you need to calculate two more quanitities, {{{sigma[x]}}} and {{{sigma[y]}}}
where
{{{sigma[x]=sqrt(sum((x[i]-x[a])^2)/N)}}} 
or
{{{sigma[x]=sqrt(Sum_of_column_5/N)}}} 
and
{{{sigma[y]=sqrt(sum((y[i]-y[a])^2)/N)}}}
or
{{{sigma[y]=sqrt(Sum_of_column_6/N)}}} 
and 
N is the number of data points, 9.

The coefficient, {{{r^2}}}, is then calculated from
{{{r^2=((1/N)*sum((x[i]-x[a])(y[i]-y[a]))/(sigma[x]sigma[y]))^2}}}
{{{r^2=((1/N)*Sum_of_column_7/(sigma[x]sigma[y]))^2}}}

EXCEL is the perfect program to do this kind of work. 
Good luck. 
Post another question if you get stuck.