start by combining your like terms, anything with the same variable... 4d and +2d, it’s important you take the sign infront of it, this will tell you what to do, ignore the variables and add, 4+2 is 6 add the variable back on 6d, take the sign in front of the other number +6 and put them together 6d+6, there are no other like terms, you cannot combine them any further
