Answer:
9 (m+5)-3(m-2)=8m+31
9m+45 - (3m + 6)= 8m+31
9m+45-3m-6=8m+31
45-6+31=8m-9m+3m
70=2m
m=70/2
Therefore m = 35
Hope u understood.............
You used the fact that you can multiply both sides of an equality by the same, non-zero constant.
So, if you start with the trivial equality
you can multiply both sides by 3:

But if
,
becomes
, which is
. So, we have

which leads to

if you recall that 
Lisa has 21 books because you add 1 to Felix because she Lisa had 1 more than him & Joe has 10 than Lisa .
32 + 2x = 84
2x = 52
x = 26
child's ticket is $26