The first part of the question simply says
Five more than three times a number
Let the number be p
Then five more than three times a number will be
5+3(number)
5+3p
It then says that it equals 21. So we’ll have to equate it to 21 I.e
5+3p=21
Arranging properly
3p+5-21=0 (21 crossed over and became negative)
Hop it’s what you’re looking for