Hi Sri,
This IR question is based on one particular graphing concept: when you draw a diagonal line on a graph, you can draw a right triangle with THAT diagonal line as its hypotenuse.
This prompt tells us that we're dealing with a square that has an area of 25, so it's side lengths all equal 5. With the co-ordinate (-2,1) as a vertex, we're looking for ANOTHER vertex that is "5 away."
The most common triangle on the GMAT with a hypotenuse of 5 is a "3/4/5 right triangle", so I'd be looking for THAT pattern among the answer choices. Is one of the numbers 3 or 4 "away" from -2? Is another 4 or 3 "away" from 1?
"-5" is "3 away" from -2.
"-3" is "4 away from 1.
So, [spoiler](-5,-3) is the vertex.[/spoiler]
GMAT assassins aren't born, they're made,
Rich