Ignore the blue and 13. but i don’t understand dividing the polynomials. it says to complete the problem.

OK the next step is to multiply 3r -1 by -7r
this gives us -21r^2 + 7r  and we write it below -21r^2 + 25r
and subtract 
This gives us  18r and we bring down the -6   to give
18r - 6

Finally we divide  18r by  -3r   to give  -6 then  add - 6 to the r^2 - 7r on the top 
then multiply -3r - 1 by -6 to give 18r - 6
subtraction then leaves nothing.

so the answer is r^2 - 7r - 6

Main idea on left, details on right