Rational expressions are considered to be undefined when the denominator is zero. The expression you have given is

(It simplifies to (-2/7)y^2 - 2.)

Perhaps your question is intended to be about the expression

-2y/(7y-2)

This expression is

7y-2 = 0

y = 2/7

*defined*for all values of y.(It simplifies to (-2/7)y^2 - 2.)

Perhaps your question is intended to be about the expression

-2y/(7y-2)

This expression is

*undefined*when7y-2 = 0

y = 2/7