Use Pythagorean theorem:
9i-j = sqrt (9^2 - 1^2) = sqrt(81-1) = sqrt80
now divide both terms in V by that:
u = 9/sqrt(80)i - 1/sqrt(80)j
see attached picture:
long way
12 x 11 x 10 x 9 x 8 x 7 x6 x5 x4 x3x2/ 10x9x8x7x6x5x4x3x2x
but you can shorten that by crossing out identical numbers so you get 12 x 11
12 x 11 = 132 combinations
Take away 1 from either side so the equation becomes 63=3x then divide both sides by 3 so it becomes 21=x which is your answer (option A)
3x-4= 6x-19 » -4+19 = 6x-3x » 15 = 3x » 15/3 = x » x = 5