A good answer might be:

d = ( -12.4,  14.8,   0.0 )T
e = (   6.2, -10.2,  17.0 )T
f = (   6.2,  -4.6, -17.0 )T
    ----------------------
    (   0.0,   0.0    0.0 )T

Yet More Practice

QUESTION 13:

Find x, y, and z so that the following is true:

a = ( 8.6, 7.4, 3.9 ) 
b = ( 4.2, 2.2, -3.0 ) 
c = ( x, y, z ) 

a + b + c = 0