Can some one help me with 5 ii) a)?

Here is what i've done.

Trailer:

fx = P-250gsin3-150=250a

fy = R-250gcos3=0

Car:

fx = 980-900gsin3-600-P=900a

fy = R-900gsin3=0

so therefore

980-900gsin3-600=P+900a

-250gsin3-150=250a-P

right? simultaneous equation then solve for P and a.

which comes out as:

a=-0.313 ms^-2 (3s.f.)

P=200N

but the answer from the mark scheme says P= 200N but a = 0.713ms^-2 which suggests the equation should be

980+900gsin3-600=P+900a

+250gsin3-150=250a-P

can someone explain what i've done wrong and which one is right.

Sorry for being quite long. THanks for your help.