Algebra

$a^3 + 3ab^2 = 679 ,3a^3 - ab^2 = 615$

$10a^3 = 3\cdot615+679$

$10a^3=2524$

$a = \sqrt[3]{252.4}$

Uhh this doesn't look right. Maybe a moderator check? If it is, @Kittykat you can use this to find a-b, just plug in