-10/8 and + 15/8. Those should work.
A title could be the effect of the amount/number of days on the amount/number of bacteria. For the 2nd The IV is temperature, the DV is Enzyme Activity and a title could be The effect of Temperature on Enzyme activity.
(4x - 8) = 80/2
4x - 8 = 40
4x = 48
x = 12
answer
x = 12