What is the gradient of the blue line?

Need on this one asap im almost done with this subject

Abacteria population is tripling every hour. by what factor does the population change in 1/2 hour? select all that apply

Aclassmate thinks that solving a system by graphing gives an exact answer when the lines appear to cross at a grid point, but only an approximate answer when they don't. explain why this isn't true.