Answer:
An expression f(x) is undefined when we have a zero in the denominator,
if our expression is
f(x) = 1/(x +7)
then the denominator is x + 7, and this is only zero when x = -7.
Then, we have that our expression is defined for all the real values except x = -7
we can write this set as:
D = {x ∈ R ∧ x ≠ -7}
(which says "x is a real number and x is different than negative seven")
