For the area of triangle ABC, actually it's just 0.5 x 6 x 3.5 = 10.5

Thank you! But i just don't get why for the gradient of AB i can't take 2-(-1) instead of what you did, which is -1-2 because the gradient i get is a negative

Yes you can do that too, but then the denominator will be 0 - (-4) and you should end up with the same answer.