Answer:
21
Step-by-step explanation:
I'll represent semi perimeter as s
What we know:
s-A=8
s-B=7
s-C=6
A+B+C=2s
we can substitute all the sides in the equation
s-A=8
s-8-A=0
s-8=A
Do this for B and C
A+B+C=2s
(s-8)+(s-7)+(s-6)=2s
3s-21=2s
-21=-s
s=21
