calculate the percentage of each element in potassium molybdenum (V) cynide?

The cyano complex of molybdenum(V) is [Mo(CN)₈]³⁻.
Hence, potassium molybdenum(V) cyanide is K₃[Mo(CN)₈].

Refer to: https://www.webelements.com/
Molor mass of K = 39.10 g/mol
Molar mass of Mo = 95.95 g/mol
Molar mass of C = 12.01 g/mol
Molar mass of N = 14.01 g/mol
Molar mass of [Mo(CN)₈]³⁻ = 39.10×3 + 95.95 + 12.01×8 + 14.01×8 = 421.41 g/mol

% of K = (39.10×3/421.41) × 100% = 27.84%
% of Mo = (95.95/421.41) × 100% = 22.77%
% of C = [12.01×8/421.41] × 100% = 22.80%
% of N = (100 - 27.84 - 22.77 - 22.80)% = 26.59%