|[3a-b]*[a-2b]| = | [3a,a] - [3a,2b] - [b, a] +[b,2b]| =

= |3*0 - (6)*[a,b] - [b,a] +2*0| = | - (6)*[a,b] + [a,b] | =

=  | - (5)*[a,b] | =|-5*|a| *|b|*sin(π/2)  | =    | -5*3*4*1| = |-60| = 60

( в решении 0 обозначен нулевой  вектор)  ,