Answer :
Since the population grows following a linear model, this means that we can write the relationship between the population and the time in a linear equation.
The standard form of a linear equation is:
[tex]y=mx+b[/tex]Where:
m = slope
b = y-intercept
Also, given two points, P and Q, we can find the slope of the line that connects them by:
[tex]\begin{gathered} \begin{cases}P={(x_P},y_P) \\ Q={(x_Q},y_Q)\end{cases} \\ . \\ m=\frac{y_Q-y_P}{x_Q-x_P} \end{gathered}[/tex]The problem tells us that at week 0, the population is 3, and at week 8 the population is 51. Those are two points that we can call:
P = (0, 3)
Q = (8, 51)
Now, we can calculate the slope:
[tex]m=\frac{51-3}{8-0}=\frac{48}{8}=6[/tex]And since the y-intercept is the value of y when x = 0, the y-intercept is the population at week 0, b = 3
Then:
[tex]P_n=6n+3[/tex]Is the explicit formula for the beetle population after n weeks.
Now, to find after how many weeks the beetle population will be 165, we substitute in the equation P = 165:
[tex]165=6n+3[/tex]And solve:
[tex]\begin{gathered} 165-3=6n \\ . \\ n=\frac{162}{6}=27 \end{gathered}[/tex]Thus, after 27 weeks the population will be 165.