To find the area of a circle, you do π × diameter (in this case 10). They have told you to use 3.142 as π, so you do 3.142 × 10 = 31.42. Because it's a semi-circle, you need to halve 31.42 to get 16.21, which is the answer
The coordinates of point C can be found as the weighted average of the endpoint coordinates. The weights are the reverse of the relative segment lengths.
For AC : CB = 3 : 1, we have ...
C = (A +3B)/(1+3) = ((-1, 0) +3(3, 8))/4 = (-1+9, 0+24)/4