Hello!
Use the Big X method, since the leading coefficient is greater than 1
Now, let me clarify this a little.
First of all, we should multiply 3 times 18:
54
Now, think of 2 numbers that multiply to 54 and add to 15.
The numbers are 6 and 9.
Now, split the middle:
3x²-6x-9x+54
I hope it helps!
Good luck and enjoy your day!
-SnowFlake