This is all in our syllabus and you must be able to do these kind of questions.(a) To find the angle between the acceleration and the i vector imagine that 6 and 8 were x and y coordinates respectively.
Plot them if you want on a sketch graph. Then do tan
-1 8/6 = 53.1 degrees.
(b) F = ma <----- you must know this simple formula.
Since we are dealing with magnitudes simply find the modulus of the acceleration vector. 6^2 + 8^2 = 100 Hence, magnitude of acceleration is 10 m/s^2
10*0.4 = 4 N = magnitude of F.
(c) Let x be the unknown velocity vector.
Solving for x you should get : 39i + 30j