Hello vq,
With the numeric solver input form on the screen and the desired input field selected, hit "next" to get to the next page of soft key labels. There you will find a "calc" softkey label (2nd from left). Hitting the associated softkey (f2) will take you to a stack environment where you can calculate your input. When finished, hit the "OK" softkey. (If, during the course of your calculation you have navigated away from these softkey labels, you will be prompted to return to them by hitting right shift "cont" (ON key).
If the equation you want is stored in a variable, hit the "choose" softkey from the solver input form.
An alternate method I sometimes like to employ is to build my equation to be solved on the main stack before even going to the solver. Then copy it, go to the solver, highlight the "eq" field, paste, and enter.
(to copy, you must activate interactive stack, and be sure the arrow is at the stack level you want to copy (it defaults to level 1), then right shift copy. You must then exit interactive stack (by hitting cancel or enter)).
One more thing, since you're new to the 50G...don't forget it's got a nifty (and powerful) symbolic solver as well. To use it, put your equation in level 2, the variable to be solved for in level 1, hit right shift "s.slv" (7 key) and use the "solve" softkey on the far right. Remember the symbolic solver won't work for all equations.
Best regards, and enjoy, Hal