Question: Solving simulataneous equations- for finding maxima of a function

hi everyone,

I am trying to find maxima for a multi variable function. As a first step I want to find out partial differentiation and equate each expression to zero.
 

Normally I would expect to solve these equations to get the variable value(s). But I came across a perticular function which cannot be solved as above explained.

Expression :=30*a*sin(theta)+80*cos(theta)*b
variable :=  {a, b, theta} # using indets(Expression,name)

PartialDifferentiationExpressions :={80*cos(theta), 30*sin(theta), 30*a*cos(theta)-80*sin(theta)*b}

How can I find the critical Points from these?

My approach is to find Hessian Matrix and ten find out maxima for the function(at which critical point it happens)

Note Edit: This is an example. I am writing a script which takes an equation as Input and solve for the maxima. Hence, I would like to know how to when this kind of situation arises. Thank You