It is not wrong and should be accepted (it occurs in native Romanian sources). But I think the point is to teach that "o mare lene" is a different construction which inverts the default word order for the sake of emphasis on "mare."

While we are at it, it is also worth noting that inn general, When a noun is determined by an adjective, the normal word order is noun + adjective, and the article (definite or indefinite) is appended to the noun. However, the word order adjective + noun is also possible, mostly used for emphasis on the adjective. Then, the article and the case marker, if any, are applied to the adjective instead.

• EXAMPLES of Noun + Adjective (normal order):

un student bun (a good student);

studentul bun (the good student);

unui student bun (to a good student);

studentului bun (to the good student).

• EXAMPLES of Adjective + Noun (reversed, emphatic order):

un bun student (a good student);

bunul student (the good student);

unui bun student (to a good student);

bunului student (to the good student).