Question 302057
Your teacher is right to ask what happened to the -16.
My question is how did you get 16 for the base?
It is correct as your check shows but how did you get it?

-5+11 -5-11
6 -16
so height is 6
base is 16


We discard the -16 for two reasons.
It will not work in the equation and we cannot have negative measurements.
You should have gotten the 16 for the base from 
48=1/2*b*6
48=3b
16=b
So now you make your statement. 
The height is 6 cm. and the base is 16 cm.
(You never said 6 or 16 what)
Then you make your check