1487.7 = 6.5x - 12.3*e-1.5x
Must have x>0 as the RHS is negative for x<0.
As x gets larger than zero the exponential term gets smaller and smaller, so an initial estimate for x might be to assume the exponential term is so small it's negligible. This will result in an initial guess for x as
x0 = 1487.7/6.5
$${\mathtt{x0}} = {\frac{{\mathtt{1\,487.7}}}{{\mathtt{6.5}}}} \Rightarrow {\mathtt{x0}} = {\mathtt{228.876\: \!923\: \!076\: \!923\: \!076\: \!9}}$$
With this value for x, how big is the exponential term?
$${\mathtt{12.3}}{\mathtt{\,\times\,}}{{\mathtt{e}}}^{{\mathtt{\,-\,}}\left({\mathtt{1.5}}{\mathtt{\,\times\,}}{\mathtt{228.866\: \!823\: \!076\: \!923\: \!076\: \!9}}\right)} = {\mathtt{0}}$$
It's not really 0, it's actually about 2*10-14, but it is small enough to ignore for most purposes!
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