The plant would be written down to the recoverable amount which is based on the undiscounted cash flows to be derived from both using the plant and disposing of it.
Cash flows from using the plant amount to $2,500,000 (i.e. 25 years X $100,000 per year) plus the residual value of $500,000 equals $3,000,000.
The company would therefore have to write down the plant, as the carrying value ($3,600,000) is greater than the recoverable amount ($3,000,000). The writedown would be $200,000, to $3,400,000, the fair market value.