Determine the value(s) for which the rational expression is undefined. -2y/7y-2. Can someone help with this?

Rational expressions are considered to be undefined when the denominator is zero. The expression you have given is 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 when
  7y-2 = 0
  y = 2/7