Question: Differential equations and substitution

Hi everyone--my differential equations course is using Maple; this is my first time using the program and I am a little cofused. 

In a lab component we are told to take the differential equation " dy/dx = sin (x - y) " and substitute in y = ax+b, then determine which coeffecients of a and b should be in order to obtain a solution.

I have made many attemps at this substitution, but am encountering syntax errors no matter what I do:

http://imgur.com/WVQsUQy

I know the general equation would be written in Maple as

>

..which presents no problems for me, format-wise, and generates a nice output.

However, any iteration I've tried with substituting y = ax+ b has resulted in an error like those shown in the linked image above; I'd really appreciate it if someone could explain how I would do the substitution and maybe what I'm doing wrong, syntax-wise.

Thank you for your time!