It depends on exactly what score you got on the mid-term. Assuming the 86.9 is the other 85% (0.85) of your grade:
Grade = (86.96)(0.85) + (Midterm Score)(0.15)
For example, if you got a 55 on the midterm:
Grade = (86.96)(0.85) + (55)(0.15)
Grade = 82.17