Question

if a-3b-5c, when a=13b-15, b =3c+4 and c =-1

Answer 1

a-3b-5c
13b-15-3(3c+4)-5(-1)
13b-15-9c-12+5
13(3c+4)-27-9c+5
39c+52-22-9c
30c+30
30(-1)+30
-30+30
0

Answer 2

First of all, put the value of a=13b-15, then we get
a-3b-5c=13b-15-3b-5c=10b-15-5c
put the value of b=3c+4,then we get
10b-15-5c=10(3c+4)-15-5c=30c+40-15-5c
=25c+25
put the value of c=(-1) ,then we get
25c+25=25(-1)+25= -25+25=0.