How do you calculate B1 from a regression formula?

I have this question, which essentially asks me to find B1, I know the answer is -2.28 but can't work out how to get it! Thanks in advance everyone.

" Shortly before you are making a group presentation on the testscore/student-teacher ratio results in Stock and Watson (Chapter 4), you realize that one of your peers forgot to type all the relevant information on one of your slides. Here is what you see:

^Test Score^ = 698.9 – B1 x STR .
...................... (9.47) (0.48)

R2 = 0.051, SER = 18.6

Find B1"