5 over the power if two divide 6 from what you get and that is your answer
1/2*(-1/7) = - (1*1)/(2*7)= - 1/14
(+)*(-) = (-)
Answer : - 1/14
This might help it has tons of different problems for your subject.
https://www.wyzant.com/resources/lessons/math/algebra/calculators/proportion
Part A: c for cost. c=0.31m+0.5
0.31m is the cost per minute. 0.5 is cost per call.
Part B: 0.31m+0.5=5.15 to solve we must rearrange.
subtract 0.5 from each side giving us 0.31m=4.85
divide by 0.31 giving us m=15.65