Step-by-step explanation:
17.
an addition is commutative. that means the sequence of the arguments of the operation does not matter (as long as the arguments are truly the same).
7x + 1 = 1 + 7x
1 = 1
0 = 0
is all true for any and every value of x.
so, yes, that's how I know that the expressions are equivalent.
18.
no, the case of 17 does not apply, because she changed the arguments of the addition (not just the sequence).
to check :
7x + 1 = 1x + 7
6x = 6
x = 1
this is true for exactly one value of x. but definitely not fit every value of x.
and so, these 2 existing are not equivalent. they are 2 different functions with a crossing point.
hence 
