Let's think about what we have.
$14.50 per hour for the first 35 hours.
$21.75 for each hour beyond the first 35.
If the total number of hours she works is x then x-35 is the number of hours she works after the first 35.
(14.50)(35)+21.75(x-35) = 768.50
Distribute
507.50 + 21.75x - 761.25 = 768.50
Now combine terms and solve for x.
-253.75 + 21.75z = 768.50
21.75x = 1022.25
x = 47
She worked a total of 47 hours. For the 1st 35 hours she was paid $14.50 an hour and for the extra 12 hours, she was paid $21.75 an hour.