1. Parallel to x=7 implies its a vertical line, hence it will contain all the points with a fixed X Coordinate...thus answer is x=3:D .
2. By similar reasoning, the line in this case will be horizontal with equation as y=4:D .
3. x^2+y^2 = 25 is the equation of a circle centered at the origin i.e. (0,0) and having a radius of 5...thus the given equation gives the set of points lying outside the circle(exterior to the circle):D .