[tex]\dfrac{\mathrm dw}{\mathrm dt}=1200-w[/tex]
[tex]\dfrac{\mathrm dw}{w-1200}=-\mathrm dt[/tex]
[tex]\displaystyle\int\dfrac{\mathrm dw}{w-1200}=-\int\mathrm dt[/tex]
[tex]\ln|w-1200|=-t+C[/tex]
[tex]e^{\ln|w-1200|}=e^{-t+C}[/tex]
[tex]w-1200=Ce^{-t}[/tex]
[tex]w=1200+Ce^{-t}[/tex]
