Q12. Lines a and b are perpendicular. This is because the products of the gradients equal -1.
Q13. Lines a b and c are parallel, because the gradients are equal.
Q14. None of the gradients are equal to each other, and none of the gradients' products equal to -1 so none of them are parallel or perpendicular.
Q15. Lines a and c are parallel (see question 13).
