You should calculate saturation vapour pressure (E) in given temperature from it, and next find actual vapour pressure (e) using relative humidity.
In the next step you should transform the Magnus formula to obtain T(E) relation, and then calculate temperature, with assumption, that previously calculated e value is equal to E in the transformed Magnus formula.
If you know the temperature & relative humidity in your space you can visit the following reference to calculate the dew point. and also for the correlation you can visit "How to use" section, or refer to common correlations.
You can use the weathermetrics package in R, which has the " humidity.to.dewpoint " function. The input of this function is relative humidity and air temperatures.