I have used this functionality in the past, but have forgotten the
usage -- can someone please help?

Lets say a result is dependent on a number of variables:
f is a function of variables a,b,c,d...
Lets say I want f to be 15. What should be the value of "a" (assuming
other variables are held contstant), such that f comes out to be 15?
Again, I cannot remember how to do this, but I do remember that I had
used this functionality few years back.

Thanks for all inputs.