4c squared + 20c?

You cannot add unlike terms. 4c² + 29c is in simplest form

You cannot add variables with different exponents...

4c^2+20c

You could look at it like:

2c(2c)+20c

2c(2c)+2c(10)

2c(2c+10)

4c(c+5) which you could have also found by simply factoring...

4c^2+20c (gcd is 4c so)

4c(c+5)

4c [ c + 5 ]

36c