I’m confused on math ( combining like terms ) ?

3x and 5x are “like terms.” to combine them, just add them together: 8x.

you cannot combine something with a variable to something without, such as 5 + 10x. this can’t be combined.

however, for this equation:
3y + 6x - 2y • 5 = 65 + 6b

you have to notice that not all of them are the same variable. you can only add and multiply the same variable. so you can do 3y - 2y. that will be y. than, you will have:

y + 6x • 5 = 65 + 6b

you can combine 5 and 65 since they are like terms. combining them would make:

y + 6x = 13 + 6b.

it cannot be simplified further.