Please help me identify the terms and like terms

The terms are (4.2x), (-3.9), (-5.1x), (6)

like terms are terms that represent the same variable.

-3.9 & 6 both do not represent any varibles, they are just numbers, so they are like terms

4.2x & -5.1x both represent (x) so they are like terms.

to collect like terms, you create a smaller equation, just for them.
so you equations are;
-3.9+6
&
4.2x-5.1x

note: you know to add the 6 as all positive numbers have a + infront, it is just not written.

-3.9+6=2.1
4.2x-5.1x=-1.1x

therefore your equation after collecting like terms is 2.1-1.1x.

Hope this helps :)