You push a desk with 245 N, but the desk doesn't move due to its friction with the ground. What is the magnitude of the friction force felt by the desk?

If the desk doesn't move, then it's not accelerating.

If it's not accelerating, then the net force on it is zero.

If the net force on it is zero, then any forces on it are balanced.

If there are only two forces on it and they're balanced, then they have equal strengths, and they point in opposite directions.

So the friction on the desk must be equal to your 245N .