To find an average you add the data values and divide by how many value you have. Since we don't know the one cubs weight we'll use a variable. Set up an equation to find average using x as the weight that is unknown.
Now solve.
(294 + 268 + x )/3 = 288
Multiply both sides by 3 to clear the fraction.
(294 + 268 + x) = 864
Add the left side.
562 + x = 864
Subtract 563 from both sides
x = 302