Verified answer

Guess and Check always works but for easy problems like this, just solve in your head

2 x 200= 400 + 2d

you need -60 to get to 340

so 2(-30) = -60. (-30 is the replacement of "d")

therefore d= -30

So 340 = 400 + 2d, and you would have to subtract 60 from 400 to get 340. That means that 1d would have to be -30.