Yes. If you are comfortable with algebra, You could instead use the constraint x = 0.1Y -2z. This will let you calculate the demand for x,z in terms of the amount of any income level (Y). Then just substitute Y=200 and Y=250 into your demand function to help you answer the rest of the question.From what I can tell, the current optimization (taken by setting the constraint as x=20-2z and plugging it into the utility function, taking the derivative, and setting it equal to zero) is x= 13.34 and z= 3.33