Remember that the gradient is the rise over the run.
If a line goes right and upwards it has a positive rise and a positive run +/+ = +, hence positive gradient.
If a line goes left and upwards it has a positive rise and a negative run +/- = -, hence negative gradient.
If a line goes right and downwards it has a negative rise and a positive run -/+ = -, hence negative gradient.
If a line goes left and downwards it has a negative rise and a negative run -/- = +, hence positive gradient.
The first one (6a) is easy because you are looking for a line with a negative gradient and there is only one line (B) which has this property.
A and D have positive gradients, E has a 0 gradient and C has an undefined gradient.
For the second one (6b) we are looking for a line with positive gradient, this means it can be on of B, C, D and E (A has an undefined gradient).
To find the value of the gradient take two points (x
1, y
1) and (x
2, y
2) and use the formula:
You'll find that if you look at line E and take the points (x
1, y
1) = (4,1) and (x
2, y
2) = (2,-5) that it has gradient 3.