We must find 2 line equations, a line equation for the red line and another line equation for the blue one.
For the red line, we have 2 points,
[tex]\begin{gathered} (x_1,y_1)=(0,25) \\ (x_2,y_2)=(700,81) \end{gathered}[/tex]by means of these point and the slope formula
[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]we have
[tex]\begin{gathered} m=\frac{81-25}{700-0} \\ m=\frac{56}{700} \\ m=0.08 \end{gathered}[/tex]hence, the form for the red line is
[tex]f(t)=0.08t+b[/tex]Now, the y-intercept b can be obtained by susbtituying point (0,25), that is
[tex]\begin{gathered} 25=0.08(0)+b \\ \text{hence,} \\ b=25 \end{gathered}[/tex]and the red line equation is
[tex]f(t)=0.08t+25[/tex]Now, lets continue with the blue line. In this case, their points are (0,4) and (700,81). Hence, the slope is
[tex]\begin{gathered} m=\frac{81-4}{700-0} \\ m=\frac{77}{700} \\ m=0.11 \end{gathered}[/tex]and the y-intercept is b=4. Finally, the blue equation is
[tex]g(t)=0.11t+4[/tex]