You simply compare the values of !(y1 & y2) with the values of x. If they are the same, the resulting value is 1 (because they are equivalent), otherwise the resulting value is 0 (because both parts are not equivalent). Or you can simply rewrite it as:

a <-> b = a&b | !a&!b = (a->b) & (b->a) = (!a|b) & (!b|a)

All four formulas describe the same Boolean function, which calculates the equivalence of two values a, b.