The equation is: [tex] 3^{x} =5x-1[/tex]
Then, to solve this on a graph using a system of equations you do:
[tex]y = 3^x[/tex]
and
y = 5x - 1.
Then you plot the both equations.
The first equation is exponential (a growing curve line) while the second is a straight line.
Then you will find graphically the point or points where those lines intercept each other.
So, you can read the x and y - coordinates of the intercection points in the graph.
x-represents the number of week and y the number of flies.
May be you find two points which means that there are two solutions. If you do not have any other restriction both points are valid. Any way check if there is other restriction that means that you have to discard one of the solutions.