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Abbreviated @Emmanuel answer

Ra=0.5; 
Rb=3.0; 
ta=pi/2; 
var('x tb'); 
sol = solve([Ra*cos(ta)+Rb*cos(tb)==x, 
             Ra*sin(ta)+Rb*sin(tb)==0.0],x,tb,
             algorithm='sympy') 
sol

[{tb: -0.167448079219689, x: 2.95803989154981},
 {tb: 3.30904073280948, x: -2.95803989154981}]