Explanation:
Two lines that are perpendicular to each other has -1 as the product of thir gradient.
Since the gradient of the line y = 3 - x is -1, the gradient of the other line must be 1.
So y = x + c.
When x = 7, y = 1. 1 = 7 + c, c = -6.
Hence the equation of the line is y = x - 6.