I agree with ooosh, you must first try the rxn yourself first. still i'm giving little mechanism.
1) O takes pi bond with him, CH3- attacks C with O.
2) -OC2H5 leaves taking pi elec. with him as OC2H5-
3) C=O reforms.
it forms a ketone which is again attacked by grignard to form tertiary alcohol.
tell me if you need help on attack of grignard on ketone.