Right...that confused me since I worked it out as the following:
Taking the piece of card to be a rectangle of width 'c' and length 'b'. (It helps to draw a diagram). Taking a square of length 'a' from each side means the remaining length is b-2a and the remaining width is c-2a.
The volume is now a(c-2a)(b-2a), and, when expanded, gives:
abc-2a²c - 2a²b + 4a³
Now, b and c are constants, only 'a' is variable. Using differentiation to find the maximum volume possible (this is an AS/A-level technique)...remembering b and c are constant:
dV/da = bc - 4ca - 4ba + 12a².
This is max (or min) when dV/da = 0.
bc - 4ca - 4ba + 12a² = 0.
Since 'b' and 'c' are constant, we have a quadratic in 'a' so we can solve:
12a² - (4b+4c)a + bc = 0
Taking the coefficients '12', '-(4b+4c)' and 'bc' to plug into the quadratic formula known for GCSE gives:
b+c plus or minus the square root of [(b-c)² + bc] all over 6. Clearly now, whatever value of 'b' and 'c' can take, the value of 'a' to maximise will be completely different.
Hope this helps....maybe not.
Kirk