Someone in Microsoft’s Answers forum wrote that the Microsoft Math Add-in doesn’t seem able to handle equations like this one:

total BTUH=CFM*(THS1-THS2)*4.5

Indeed, when I insert that equation into an equation container in Word 2010, then choose Compute – Solve, this is what I see:

The Math add-in does not handle multi-character variable names done in that way. Instead, it treats “total BTUH” as t*o*t*a*l*B*T*U*H. What fun!

An awkward but possibly-acceptable work-around is to substitute single-letter variables,

b=x(w-z)4.5

Now, Math offers the following choices:

It will also let you graph in 3D:

But, note that the choices of letters are important, since Math has some pretty set ideas about where x, y, and z belong in a 3D graph, as well as what specific letters mean. If I arbitrarily assign different letters, it ranges from not working at all to giving entirely different graphs.

Another route is to use _ to create multi-character variable names. If we try to let the variables resemble the original names, such as choosing:

b_tuh=c_fm(t_hs1-t_hs2)4.5

Solve now works correctly:

But, now all of the choices for Graph are grayed out:

And, as usual… YMMV (your mileage may vary).