I have these two equations:

D0 - dp

S0 + sp

find value p, such that supply and demand is equal.

My assumption is to set p - p = 0, so that the equation is

D0 - dp - S0 + sp = 0

I solve to get

p = D0 - S0/ d - s,

but the answer in the book is

p = D0 - S0/ s + d,

I am not able to understand how they got this answer.