I'm assuming that the second terms in each equation should involve an exp,

Subtracting [2] from [1] gives an expression for y in terms of x:

Subbing this expression for y into [1] and reorganizing gives

This can be simplified using the following two identities.

Unfortunately, this equation is transcendental in x, which means it cannot be solved analitically. The solution for x (for which there will be an infinite number due to the periodic nature of the trig functions, as long as P isn't too large compared to the Qs) can be found either graphically or numerically. For a graphical solution, you plot the right-hand side of the equation and see where it crosses the x-axis (these will be the points where the function on the right is equal to zero, satisfying the equation). For a numerical solution check out root finding methods like the bisection method. Once you have values for x you can plug them into the above expression for y.