Hi MOUSUMI,
I did not encountered any issue while working on this task. You need to double-click on the canvas and type constant to find the Constant block, and add it to the model. Set its value to 10*0.25. Then, use an Add block to add it together with the x_kminus1 signal. This is what is required to be done in Task3.
The final model needs to be look something like this.
Kindly let me know if the issue still persist.