x²-14x+49-y²

Factor the first three terms x²-14x+49 as (x-7)(x-7) and then write as (x-7)²

(x-7)²-y²

That factors as the difference of two squares:

[(x-7)-y][(x-7)+y]

Remove the inner parentheses:

[x-7-y][x-7+y]

Change the [] to () and write the letters first
in each:

(x-y-7)(x+y-7)

Edwin