help me! tolong dibantu ya Kaka,mohon kerjasamanya

P(2, -1, -3), Q(-1, a, -11) dan R(4, -3, -2)
PQ = q - p = (-1-2, a-(-1), -11-(-3)) = (-3, a + 1, -8)
PR = r - p = (4-2, -3-(-1), -2-(-3)) = (2, -2, 1)
=> |PR| = √(2^2 + (-2)^2 + 1^2) = √(4 + 4 + 1) = √9 = 3

Proyeksi vektor PQ pada PR = -4i + 4j - 2k
(PQ.PR)/|PR|^2 . PR = -4i + 4j - 2k
(-3(2) + (a + 1)(-2) + (-8)(1)) / 3^2 . PR = -4i + 4j - 2k
(-6 - 2a - 2 - 8)/9 . PR = -4i + 4j - 2k
(-16 - 2a)/9 (2i - 2j + k) = -4i + 4j - 2k
(-16 - 2a)/9 (2i - 2j + k) = -2(2i - 2j + k)
(-16 - 2a)/9 = -2
-16 - 2a = -18
-2a = -2
a = 1