Answer:
-3.0
Explanation:
First, we open the bracket and take note that:
[tex]+\times-=-[/tex][tex]\begin{gathered} -1.8+\left(-1.2\right)=-1.8-1.2 \\ =-3.0 \end{gathered}[/tex]Next, we determine Tom's mistake.
[tex]\begin{gathered} 1.8+(-1.2)=1.8-1.2 \\ =0.6 \end{gathered}[/tex]Therefore, the mistake Tom made was that he ignored/forgot the negative sign before 1.8.