If the rate of change of velocity of a body is said to be constant, what is the acceleration of the body

the entire statement is true and correct.  

there's no diagram, nothing is illustrated, and no question is asked.

see the picture for the vector

the magnitude of the vector by pythagorean theorem

r = √( (-10)^2 + (3)^2 ) = √109

magnitude = √109 units

the direction angle between vector and x-axis is

tan^(-1)(3/10) = 16.699

so the direction is approximately 16.7 degrees north of west