When we say "overtime at time and a half", we can either multiply the pay by 1.5 and use the given hours, or we can multiply the hours by 1.5 and use the given pay. It might be easier to do the latter.
... ((38 h) + 1.5×(6 h)) × ($15/h) = (47 h)×($15/h) = $705
Miss Martinez's gross pay will be $705.