With the alto note, the interval between B below middle C and A in the treble clef would be an minor 7th, so just think what would be a minor 7th below the B below middle C? It would be C# in the bass clef.
For the soprano, the interval between B below middle C and E in the treble clef would be an octave and a perfect fourth, so an octave and a perfect fourth below the B below middle C would be an F# in the bass clef.
Make sense?