Find the sum of the vectors `veca=hati-2hatj+hatk,vecb=-2hati+4hatj+5hatkandvecc=hati-6hatj--7hatk`.

Explanation

`h=("volume of parallelepiped")/("Area of base")`
`=([vec(A)vec(B)vec(C)])/(|vec(A)xxvec(B)|)=(|(1,1,1),(2,4,-1),(1,1,3)|)/(|(hat(i),hat(j),hat(k)),(1,1,1),(2,4,-1)|)=(4)/(|-5hat(i)+3hat(j)+2hat(k)|)=(2sqrt(38))/(19)`