There is much value in writing very clearly. Don't be afraid to write MORE. It will help you.
You're using 'x', let's go with that.
x = # of 8-hour shifts.
1. she worked 2 times as many 6 hour shifts as 8 hour shifts
2x = # of 6-hour shifts
2. she worked a total of 280 hours
Each 8-hour shiift counts 8 hours. x*8 = Hours worked in 8-hour shifts.
Each 6-hour shiift counts 6 hours. (2x)*6 = Hours worked in 6-hour shifts.
(This may seem really obvious, but look what it does for us.)
x*8 + (2x)*6 = 280
You just didn't translate the last piece.
Note: This assumes this is all she worked. If she pulled a double or a 3-hour shift, we're wrong. Only 6s and 8s on this one.
Slowly, carefully, deliberately.