How do you simplify 12x + 15x + 3?

how do you simplify 12x + 15x + 3 help kinda forgot about these over the summer..their lil warm up but i only need one example to refress my memory please & thanks!!
Update: thats what i thought thanks but what about 13x - 7y + 26x + 5y - 17 dont u have to swtich them around cuz the varibles?!?!
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