¿How do I calculate the net displacement in this problem? I understand how they got the displacement from the square but not from the triangle it

Use Figure 2.19 to (a) find the approximate displacement of the jet car over the time shown, (b) calculate the instantaneous acceleration at t = 30 s, (c) find the instantaneous velocity at 30 s, and (d) calculate the approximate average velocity over the interval shown.

This problem is more complicated than the last example. To get a good estimate, we should probably break the curve into four sections. 0 → 10 s, 10 → 20 s, 20 → 40 s, and 40 → 70 s.
Calculate the bottom rectangle (common to all pieces). 165 m/s ×
70 s = 11,550 m.
Estimate a triangle at the top, and calculate the area for each section. Section 1 = 225 m; section 2 = 100 m + 450 m = 550 m; section 3 = 150 m + 1,300 m = 1,450 m; section 4 = 2,550 m.
Add them together to get a net displacement of 16,325 m.
Using the tangent line given, we find that the slope is 1 m/s2.
The instantaneous velocity at t = 30 s, is 240 m/s.
Find the net displacement, which we found in part (a), was 16,325 m.
Find the total time, which for this case is 70 s.
Divide 16,325 m70 s∼233 m/