n m k : Nat
h : k ≤ n
⊢ n + m = n - k + m + k
n m k : Nat
h : k ≤ n
⊢ n + m - k = n - k + m
rw [Nat.sub_eq_iff_eq_add (Nat.le_trans h (Nat.le_add_right ..))]
no goals
n m k : Nat
h : k ≤ n
⊢ n + m = n - k + m + k
rwa [Nat.add_right_comm, Nat.sub_add_cancel]