If you have done the first part, the second part should be very easy.
Use the regression equation to predict noise level for the given distance of 400m.

If the noise level exceeds the preassigned value (60DB) then you can not have school within 400m

ah ok. so i just use the least squares regression line equation and just put in 400 for distance and see if it exceeds 60 dB? seems like that's the way to do it!