[tex] {a}^{2} + {b}^{2} + {c}^{2} = 21[/tex]
[tex] a + b - c = 7[/tex]
[tex]ab - bc - ac =[/tex]
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(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

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