$.$ Find x$($t$)$ if dx$/$dt$=-63$x$-12$y$,$ dy$/$dt$=420$x$+79$y$,$ and x$(0)=5,$ y$(0)=5.$ x$($t$)$ is of the form x$($t$)=$a$$ exp$(9$t$)+$b$$ exp$($c$$t$)$ for some constants a$,$b$,$ c$.$ Report a$.$

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