7. (6 points) In the laboratory, a peculiar plant was cultivated, which grows very rapidly, tripling its mass from the previous day and adding an extra 4 kilograms every day. After being cultivated for 3 days, the plant's mass reached 133 kilograms. The original mass of the plant was $\qquad$ kilograms.

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【Answer】Solution: Let the plant originally weigh $x$ kilograms. According to the problem, we have:
$$
\begin{aligned}
3 \times[3 \times(3 x+4)+4]+4 & =133, \\
3 \times[9 x+16]+4 & =133, \\
27 x+52 & =133, \\
27 x+52-52 & =133-52, \\
27 x & =81, \\
27 x \div 27 & =81 \div 27, \\
x & =3 ;
\end{aligned}
$$

Answer: The plant originally weighed 3 kilograms.
Therefore, the answer is: 3.