To work out the mean, add them all up and divide by however many values there are:
(18+54+42+(76x2)+46) / 6 = 52
To work out the median, order them from largest to smallest or vice versa and find the middlest value.
18, 42, 46, 54, 76 ,76
(46 + 54) / 2 = 50
The mode is the most frequently occurring number, so it would be 76.
Hope I helped!