We have that
[tex]78x+(20x)[/tex]When a number doesn't have a sign we just suppose that it is a plus. For example,
[tex]\begin{gathered} 70=+70 \\ 3x=+3x \end{gathered}[/tex]Therefore,
[tex]78x+(20x)\rightarrow78x+(+20x)[/tex]Remember that if we multiply equal sings, we get a plus. Thus,
[tex]\begin{gathered} 78x+(+20x)\rightarrow78x+20x\rightarrow98x \\ \text{The answer is}\colon \\ 98x \end{gathered}[/tex]