why am I getting the error “Incorrect number or types of inputs or outputs for function ‘solve’.”

my code is;

syms d;
s=solve('7*d + 3*e=71','d + 2*e=20');
s=[s.d s.e];
disp('1 dollar = ');
s(1)
disp('Eullar');

I am getting the error:

Incorrect number or types of inputs or outputs for function ‘solve’.

Error in HW10 (line 2)
    s=solve('7*d + 3*e=71','d + 2*e=20');

The qustion I am solving is:

"Seven dollars and 3 Euros make 71 Eullars.
    One dollar and 2 Euros make 20 Eullars.
    find out the value of a dollar in terms of Eullars"

NOTE:-

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For equality inside a symbolic expression, use double equal signs (==) —

syms d e 
s=solve(7*d + 3*e == 71,d + 2*e == 20);
s=[s.d s.e];
disp('1 dollar = ');

1 dollar =

vpa(s(1),3)

ans = 7.45

disp('Eullar');

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why am I getting the error "Incorrect number or types of inputs or outputs for function 'solve'."

why am I getting the error "Incorrect number or types of inputs or outputs for function 'solve'."