We break apart the first numer:
10+8
the second:
20+1
and add like this:
18+21=10+8 + 20+1
now, we can mix up the order:
18+21=10+8 + 20+1= 10+20 + 8+1
add the parts:
18+21=10+8 + 20+1= 10+20 + 8+1=30+9=39
Just factor. y = (x-1)(x-1)
I believe the answer is D
