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,

by means of these point and the slope formula

we have

hence, the form for the red line is

Now, the y-intercept b can be obtained by susbtituying point (0,25), that is

and the red line equation is

Now, lets continue with the blue line. In this case, their points are (0,4) and (700,81). Hence, the slope is

and the y-intercept is b=4. Finally, the blue equation is
