How is secretariat related to seattle slew?

Secretariat was born before Seattle Slew, so really, it should be said the other way around.

But anyway, Secretariat and Seattle Slew were indeed related, through their sires. Seattle Slew's sire, Bold Reasoning, was the grandson of Bold Ruler, who was Secretariat's sire. They are also related through the bottom half of the pedigree, through Princequillo, who is Seattle Slew's Great-great-grandsire, and Secretariat's damsire.