Answer:
It helps if you know the definition of 'average' and can write it down.
Avg = 1/2 (MAX + min)
Eliminate the parentheses on the right side:
Avg = 1/2MAX + 1/2min
Subtract 1/2MAX from each side:
Avg - 1/2MAX = 1/2min
Multiply each side by 2:
2Avg - MAX = min
which is exactly what you're looking for.