You should find that the figure 8 knot has 4 crossings, 3 Seifert circles and 1 component which gives a knot genus of $(2+4-3-1)/2=1$. Given that knot genus is a natural number, and the trefoil and figure 8 knots are not trivial, we can deduce that they both have genus 1. If they were not prime, then there would exist two knots of positive integer genus (as the unknot is not prime), whose geni add to give 1, by Seifert's Theorem. This is clearly a contradiction.

You should find that the figure 8 knot (with its standard knot diagram) has 4 crossings, 3 Seifert circles and 1 component which gives a knot genus of $(2+4-3-1)/2=1$. Given that knot genus is a natural number, and the trefoil and figure 8 knots are not trivial, we can deduce that they both have genus 1. If they were not prime, then there would exist two knots of positive integer genus (as the unknot is not prime), whose geni add to give 1, by Seifert's Theorem. This is clearly a contradiction.