A = 2i - 3j + k
b = 3i + j - 4k
a - b = (2 - 3)i + (-3 - 1)j + (1 + 4)k
a - b = -i - 4j + 5k
2a = 4i - 6j +2k
2a + b = (4 + 3)i + (-6 + 1)j +( 2 - 4)k
2a +b = 7i -5j -2k
скалярное произведение
(a - b)(2a + b) = -1·7 + (-4) · (-5) + 5 · (-2) = -7 + 20 - 10 = 3