6n^2-5n^2-7n^2 simplify the expression

For how answer my question this will be the last question can you answer in math subject

What is the missing statement in step 4? ? rts ? ? vtu and ? rtu ? ? vts ? rts ? ? rvs and ? rtu ? ? stv ? vrs ? ? vru and ? usr ? ? usv ? vur ? ? vus and ? uvs ? ? sru

Out of 50 students surveyed, 16 have a dog. based on these results, predict how many of the 280 students in the school have a dog.