from Wikipedia:
= (K+L)(K+M)(L+N)(M+P)(N+Q)(P+Q)
= (K+LM)(N+LQ)(P+MQ)
= (KN+KLQ+LMN+LMQ)(P+MQ)
= KNP + KLPQ + LMNP + LMPQ + KMNQ + KLMQ + LMNQ + LMQ
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K * N+LQ + LM*(N+LQ)
K * N+LQ = KN+KLQ and LM*(N+LQ) = LMN+LMQ
for the : (KN+KLQ+LMN+LMQ)
for the fourth line multiply the third line by P and then by MQ from (P+MQ)