An accidental sign consisting of two flat symbols ♭that lower a note by two half steps two semitones. The double flat symbol alters the pitch of the note to which it is attached as well as any subsequent occurrence of the same note identical line or space in the same measure.
It's pretty straight forward, you have the number 4 that is added to the rest of the expression because sum is addition, and the product of 2 numbers is multiplication so it would make 3(x) or 3x, which results in 4+3x.
