If I'm right, the answer is -4b + d.
"Diminished by" means to subtract. So putting both of these equations into one would read:
a - 2b+ 3c - a - 2B - 3c + d
So group like terms and signs.
Negatives and positives cancel out, so the a's and the 3'c cancel.
Now the equation is -2b - 2b + d.
Grouping like terms, your final answer is -4b + d
