a) You'll notice from "c)" that the distance the object travelled was 75m, so you get [tex]V_a_v_e_r_a_g_e = 75m/15s = 5 m/s[/tex].
b) [tex]a = dV/dT [/tex], so [tex]a = (0-10)/15[/tex] and then you get [tex]a = -2/3 m/s ^{2} [/tex].
c) [tex]S = S_0 + V_0T + 0,5aT^2[/tex] and you get [tex]S = 10T - (2/6)T^2[/tex] and you plug in [tex]T = 15s[/tex] and you'll get [tex]S = 75m[/tex]