def forward_sub(L, b):
    """Given a lower triangular matrix L and right-side vector b,
    compute the solution vector y solving Ly = b."""

    y = []
    for i in range(len(b)):
        y.append(b[i])
        for j in range(i):
            y[i]=y[i]-(L[i][j]*y[j])
        y[i] = y[i]/L[i][i]

    return y



    return x