`y = 5/(cos^2 x)` represents a relation between y and x. This is not something that can be solved, unless x or y has to be determined for a particular value of y or x repectively.
Examples of such equations are `5 = 5/(cos^2x)` . The solution to this is `cos^2x = 1`
=> `cos x = 1` and `cos x = -1`
=> `x = n*2*pi` and `pi + n*2*pi`
Similarly, an example of an equation where the value of x is given is: `y = 5/(cos^2 pi)`
=> y = 5/1
=> y = 1