"You should compute their values modulo prime P."
(-1235+1+1+1+1+1)%1237=-1230, but answer 7. I do not understand =(

According to famous mathematical convention we can write
a % b = (n*b + a) % b as n tends to infinity. So, -1230 % 1237 is equivalent to (-1230 + 1237) % 1237 == 7