When two lines are perpendicular, their slopes are negative reciprocals.
That means that the slope of one line is -1/(slope of the other line).
You have the slope of one line. It's -3/7 .
So the slope of any line perpendicular to that one is 7/3 .
Now you know the slope of the new line, and you know one point on it.
Slope = 7/3
Through the point (-7, 6)
I'm pretty sure you know how to do the rest.
The trick was knowing how to find the slope of the new line.