Notice that T is a linear map of R^3 to R^2. No one such linear map from R^3 to R^2 can be invertible. Just because many different elements of R^3 map into the same element of R^2. For example, many different elements of R^3 map to 0 (zero) element of R^2. More concretely, all the vectors of the form (x,y,z) = (x,-x,-x) for any real value of x map into 0 (zero) by the map T. It means that the inverse mapDOES NOT EXIST.