And equation that equals 23 that has a negative in it but the answer is positive

Which property is illustrated by the following statement? if hxa rgi, then rgi hxa.

Amerchant has coffee worth $60 a pound that she wishes to mix with 50 pounds of coffee worth $90 a pound to get a mixture that she will sell for $70 a pound. how many pounds of the & 60 coffee should be used?

Solve each quadratic equation by factoring and using the zero product property. x^2 + 18x = 9x