how do you find the major key from the flat major signature

For a flat major key signature, the rule is simple: look at the second- to-last flat in the signature, and that note is the major key. For example, B♭ and E♭ means B♭ major, and B♭, E♭, A♭, D♭ means A♭ major.

Fast rule

• 1 flat: F major.
• 2 or more flats: use the second-to-last flat as the key name.

Example

If the signature has B♭, E♭, A♭, D♭, G♭, the second-to-last flat is D♭, so the key is D♭ major.

Easy memory aid

Flat keys go in this order: B♭, E♭, A♭, D♭, G♭, C♭, F♭, and the key is usually named by the penultimate flat.

TL;DR: For flat major keys, take the second-to-last flat; if there is only one flat, it is F major.