If you insert an equal sign into a function to graph, g(Math) will throw an error and not graph your function. For example if you entered Sin[x]=3 over the range of x from x to 8 and y from 0 to 15, g(Math) will interpret this as y=3 (completely ignoring the Sin[x] part).

Instead you would have to enter Sin[x], 3 and then the answer would be the intersection of these 2 functions. (I am working on having g(Math) find the intersection of these functions for a future version).