For straight line graphs,
where is the slope and is the -intercept.
Comparing to the above, we can see that the for our straight line, the slope is .
We know that the slope, , of a line passing through 2 points and is given by
So, the slope of the line passing through and is
If 2 straight lines, and , having slopes and are perpendicular, then .
Let the required line have slope .
But the slope of the line is . So, we can say
Solve for .
So, the value of a is .