Lets see.
In the insert drop down box is special characters
the integra sign is 3rd row 9th across ∫ the rest I would just enter by normal typing.
OR
if i enter int(3x) int the maths formula input box I get ∫(3×x)dx
OR
if you use the LaTex Formula input box you can type in \int(3x)dx and you will get ∫(3x)dx
Lets see.
In the insert drop down box is special characters
the integra sign is 3rd row 9th across ∫ the rest I would just enter by normal typing.
OR
if i enter int(3x) int the maths formula input box I get ∫(3×x)dx
OR
if you use the LaTex Formula input box you can type in \int(3x)dx and you will get ∫(3x)dx