Strictly speaking, what you have is NOT correct but only because you are missing parentheses. The derivative is given by the limit of ((x+h)sinx+h- xsinx)/h. What you wrote would be (x+y)sinx+h- (xsinx/h). Even clearer would be ((x+h)sin(x+h)- xsin(x))/h. Now I would recommend that you use "-a+ a" to isolate the changes:

((x+h)sin(x+h)- xsin(x+h)+ xsin(x+h)- xsin(x))/h and separate into (((x+h)- x)/h)sin(x)+ x((sin(x+h)- sin(x))/h.

The first you should recognise as the derivative of x and the second as the derivative of sine.