Hi,
If you played pb4 then black can play cxb4+.
So that wouldn't checkmate the King so ra6 is correct.
aha, I didn't notice. Thanks!
Go thru the basics of checks, captures & threats
There are two checks (Ra6+ & b4+); evaluate each check; Ra6+ is the only move where Black has no response
Don't stop with the first check option, look at all possibilities; the correct move will present itself