ada yang bisa jawab?

α tumpul
cos α = - √(1 - sin² α) 
cos α = - √6 / √7

c = b - a
c . c = (b - a)(b - a)
|c|² = |b|² - 2|a||b| cos α + |a|²
|c|² = 7 + 2√30 + 5
|c|² = 12 + 2√30 

b . b = (a + c)(a + c)
|b|² = |a|² + 2|a||c| + |c|²
2|a||c| = |b|² - |a|² - |c|²
2ac = 7 - 5 - 12 - 2√30
2ac = -10 - 2√30
ac = -(5 + √30)