By the end of the 7th grade, do students need to simplify expressions with unknowns and negative numbers such as 5x-3x=2x ?
Or do they only need to evaluate numeric expressions without unknowns (5-3=2)?
Perhaps what you are asking about is mostly contained in 6th grade, in 6.EE.3. Students apply the properties of operations to generate equivalent expressions. In the case you mention, students would use the distributive property to rewrite 5x – 3x as
(5 – 3)x, and then 2x.
In grade 7 students build on that in 7.EE.1 by including expressions with rational coefficients.