There are big long formulas for these.  But if you have a TI-83 or 84 calculator,

Press STAT then ENTER
(If there are numbers under L1, highlight L1, press CLEAR ENTER.  
Same for L1.  

Type in the first numbers under L1 2 ENTER 3 ENTER 6 ENTER 7 ENTER 9 ENTER

Type in the second numbers under L2 -7 ENTER -11 ENTER -12 ENTER -13 ENTER -14 ENTER

Press STAT
Press the right arrow to highlight CALC
Scroll to 4:LinReg(ax+b)
Press ENTER

          Xlist:L1
          Ylist:L1
          FreqList:
          Store RegEQ:
          Calculate

Highlight Calculate
Press ENTER

          y=ax+b
          a=-0.8493975904
          b=-6.813253012
          r²=0.8203086318
          r=-0.9057089112

So round off a, b, and r, to the nearest hundredth., and substitute
for a and b in ŷ=ax+b.

Edwin