you are right, I will copy my program below to avoid this kind of mistakes.

look I copy paste exactly as it is in maple because my typo mistakes may complicate all

r1:=2;
r2:=3;
r4:=7;
Gamma:=30;

                                      2
                                      3
                                      7
                                     30

theta[2]:=20;
theta[3]:=theta[4]-Gamma;

                                     20
                                theta[4] - 30

fsolve({r2*cos(theta[2]) - r3*cos(theta[3])-r4*cos(theta[4])-r1=0,r2*sin(theta[2])-r3*sin(theta[3])-r4*sin(theta[4])=0}, {r3,theta[4]});

fsolve({3 sin(20) - r3 sin(theta[4] - 30) - 7 sin(theta[4]) = 0,

  3 cos(20) - r3 cos(theta[4] - 30) - 7 cos(theta[4]) - 2 = 0}, {r3, theta[4]})

Sorry, I'm new here and with maple, sorry if my problem is something stupid

you are right, I already  assign values, to r1, r2 and r4 but I forget to mention here
my mystake