question for "the short-term,or trip,ruin formula"

BJ attack 2nd edition page 162 is the formula for calculate the bank
roll that we need for N days. page 163 is the exambel- i can underestend olmost everything tyl (.0918+.6187*.1841=.2057) this,
fom where the hell coming this results? who can i make this calculetion---N (-1.33)+e-.48 * N (.90)???

thanks in advance..!