please help!!

i have obtained the equation of the regression line for a previous question which is: y=62.4+0.663x
i had to use this to calculate the residual of a patient whose blood level was 142mg two days after a heart attack and was 116 four days after a heart attack. the explanatory variable was the patients blood level two days after a heart attack. i need to now explain why using my equation would be inappropriate to estimate the patients blood level four days after a heart attack when their blood level is 375mg two days after having a heart attack.
any ideas because im really stuck on this question thank you very much

The regression line was obtained under differing baselines. It is not applicable under changed conditions.