답안

function solution(a, b) {
    let c = [];
    const len = a[0].length;

    if (len === b.length){
        for(let i=0; i<len; i++){
            let row = [];
            for(let j=0; j<len; j++){
                let x = 0;
                for(let k=0; k<len; k++){
                    x += a[i][k]*b[k][j];
                }
                row.push(x);
            }
            c.push(row);
        }
        return c;
    } else {
        return -1;
    }
}

const a = [[1, 2], [2, 4]];
const b = [[1, 0], [0, 3]];

console.log(solution(a, b));