In matlab (and most other programming-languages) you will have to write out the multiplication-operator (* for matrix multiplication or .* for elementwise multiplication). The way you write:
Will be interpreted as the cosh(600/x)-12th element of the matrix x. Note here that 600/x would be a right-matrix-division, for elementwise division you will have to use ./.
Most likely you want:
f= (x*(cosh(600/x) -12)-108);
Then you will have a problem with the next line too, because f is now a scalar which make the function diff return an empty array, , so you will have to do something about that.
If you are very new to matlab take a trip through the on-ramp courses on the Matlab web-site.