Key idea. Use a difference of squares on the two whole brackets.

1. The identity is \(U^2-V^2=(U-V)(U+V)\). Here take \(U=2a+b\) and \(V=a-2b\).

2. Find the difference carefully:

   \[U-V=(2a+b)-(a-2b)=2a+b-a+2b=a+3b.\]

3. Find the sum:

   \[U+V=(2a+b)+(a-2b)=3a-b.\]

4. Multiply the two expressions in factorised form. This gives \((a+3b)(3a-b)\).

Remember. A complicated-looking bracket can be treated as a single quantity. Be careful when subtracting a bracket.