Nomor 6. Tolong dibantu soal kelas XII

syaraat tegak lurus
berarti sudutnya = 90
a · b = |a| |b|

a · b = (2)(p) + (-5)(-2) + (1)(4)
        = 2p + 14
|a| = √(2² + (-5)² + 1²)
     = √(4 + 25 + 1)
     = √30
|b| = √(p² + (-2)² + 4²)
     = √(p² + 4 + 16)
     = √(p² + 20)

a · b = |a| |b|
2p + 14 = √(30p² + 60)
4p² + 56p + 196 = 30p² + 60
26p² - 56p - 136 = 0
p = 2/13 [7 - 3√30]
atau 
p = 2/13 [7 + 3√30]