It helps to NOT define your known constants numerically BEFORE you do symbolic operations. As an alternative you might use the clearsym function (look that up in the help!).
But most of all: DO attach your worksheet, NOT only a picture.
You have to type "solve, DeltaT,...." in the placeholder above of the arrow similar to what you see in the picture provided by Luc. Lucs picture is from real Mathcad but this also works in Prime.
In case you still have problems getting the answer you should consider posting your worksheet instead of just a picture.