First question:
As can be seen from the picture, all coordinates in the form (n,n-1) (such as (5,4)or (3,2) have a diagonal arrow pointing towards them, going downwards and to the right, and a similar arrow pointing away from them (with the exception of (2,1))therefore, as (18,17) is in the form (n,n-1), the next point in the route will be (19,16) as this is one point across and one point down
Second question:
If Tn is the number of the triangle formed (a triangle is "formed" when either and up or an across action is then next action taken. This should be fairly obvious from looking at the diagram), U is the amount of up actions taken in the entire journey so far, D is the number of diagonal actions in any direction taken so far and A is the number of across actions taken so far, then the first 3 triangles take the below forms:
T1=1U+1D (triangle finishes on (2,1))
T2=1U+3D+1A (triangle finishes on (1,3))
T3=2U+6D+1A (triangle finishes on (4,1))
From this we can see that:
Every odd T adds 1U
Every even T adds 1A
The number of Ds is the sum of all of the numbers up to that number T (i.e T3 has 6 Ds because 1+2+3=6 and T4 has 10 Ds because 1+2+3+4=10)
As all triangles end with one of the two coordinates being a 1, we can see that (9,4) will go to (10,3), then (11,2), then (12,1)(Diagonal actions clearly go down and right here, as the Y coordinate is closer to a 1 than the x coordinate). As has been shown above, the coordinates (12,1) mark the end of the 11th triangle. Using the rules above:
U=6 as there are 6 odd numbers up to and including 11 (1,3,5,7,9,11)
A=5 as there are 5 even numbers up to and including 11 (2,4,6,8,10)
D=66 as 1+2+3+4+5+6+7+8+9+10+11=66
6+5+66=77, so 77 up, across and diagonal routes have been taken to reach (12,1). As we are only trying to reach (9,4), we can minus 3 routes to make 74 routes. This means that (9,4) is the 75th point visited