Yes,  -7x+21 is the same as 21-7x​, that is:

 -7x+21​ = (-7x) + 21 ​

Since the Commutative property​ of adding that states that 'a + b = b + a', if we put a = -7x and b = 21 we have:

a + b = (-7x) + 21  ​​

b + a = 21 + (-7x) = 21 - 7x

And that is exactly what is written above.