how can i solve this angle between vectors?

Find the angle between the vectors a and b given that a = 3; b = 5; and a-b = 7:

Answer 1

[Andrei Bobrov]

Law of cosines

a = 3; 
b = 5;
c = 7; % c = a - b

alpha = acos((a.^2 + b.^2 - c.^2)./(2*a*b));

Answer 2

[VBBV]

a = 3; 
b = 5;
c = 7;
alpha = acosd((a.^2 + b.^2 - (c).^2)./(2*a*b)) 

alpha = 120.0000