@Thomas Richard 

Thank you Mr.Thomas Richard, your answer solved  my equation, and I could plot the mode shape .

I tried to use this code in a loop in this way

v[0] := (1/5)*h

with(RootFinding):

freq := Analytic(aq, omega, re = -10 .. 300, im = -10 .. 100)

count := 0;

 for g from 1 to 10 do;

if abs(Im(freq[g]))<10^((-3)) and Re(freq[g])>0 then

count := count+1;

p[count] := omega, _n;

 end;

 end;

FirstNfreq := p[1];

SecondNfreq := p[2];

end:

 but it didn`t work.

can you help me?

@acer @Rouben Rostamian 

Hello,

Thank you.your comments solved my problem and help me.

@Rouben Rostamian  

Thank you Rouben Rostamian I did this,

 M := Matrix(4, 4, [a1, a2, a3, a4]);
               
ans := LinearAlgebra:-NullSpace(M);
       
v := ans[1];
           
M.v;
           
but M.v is like v!

@Rouben Rostamian  

Hello, Rouben Rostamian

 In my question M3 is a 4x4 matrix and b is a vector. I did your suggestion, it gives me a vector, but I`m not sure that it is true.

Thisis my code:

M3 := Matrix(4, 4, [a1, a2, a3, a4]):

Then by obtaining eigenvectors in  this equation , I must have b[0],b[1],b[2] and b[3].

Do you think that my code is true?