The original sentences are given in the simple present. To change this sentences into the simple future, we must change the conjugations of each verb. Therefore, we have the following:
Ella irá a la playa.
Va is the conjugation of the verb ir for the third person singular in the simple present and changes into irá in the simple future.
Tendré mucho trabajo.
Tengo is the conjugation of the verb tener for the first person singular in the simple present and changes into tendré in the simple future.
Tendrás muchos libros.
Tienes is the conjugation of the verb tener for the second person singular in the simple present and changes into tendrás in the simple future.
