How to get an input string from text file into a variable in shell scripting?

I have two files in my directory,

the multiply.txt file contains only a single line 3*6

I want to display the value of the expression in the multiply file using only echo and cat commands.

the runner.txt contains the shell script required

I have tried

echo $((cat multiply.txt)) #didnt work

I also tried to extract 3*6 as a string into a variable and none of the below commands seem to do the trick.

value = cat<multiply.txt # didnt work

value = cat multiply.txt # didnt work

value = $(cat multiply.txt)

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