a² + b² + c² = 21
a + b - c = 7
(a + b - c)² = 7²
[(a + b) - c]² = 49
(a + b)² - 2 * (a + b) * c + c² = 49
a² + 2ab + b² - 2ac - 2bc + c² = 49
(a² + b² + c²) + 2ab - 2bc - 2ac = 49
21 + 2(ab - bc - ac) = 49
2(ab - bc - ac) = 28
ab - bc - ac = 14
(a+b-c)^2 = a*a + b*b + c*c + 2ab -2ac -2bc
2ab -2ac -2bc = (a+b-c)^2 - a*a + b*b + c*c = 49 - 21 = 28
ab -ac -bc = 28/2 = 14
