A single equation with two variable can have an infinite number of solutions. You need to fix atleast one value. For example
y = 1.5;
x = acosh(cosh(y)*1/2)
In this case you have an expression for the solution of 'x' in term of 'y'. In general case, you can use fzero() or fsolve()
y = 1.5;
fzero(@(x) cosh(x)/cosh(y)-1/2, rand())
