$(8$ pts$)$ The graphical displays shown for this problem resulted from a model that was fitted using the mtcars data set in R$.$

model disp $+hp+wt+$qsec, data$=$mtcars$).$

a$.$ The following Partial Regression Plot was generated. Describe the variables that are plotted on the $x-$axis and on the $Y-$axis.

b$.$ What information does the blue line in the diagnostic plot below provide to the experimenter?