Haii buddy!! so.. yah firstly you read the question carefully then you can understand how to do the solution part.

building is of 80m height and a ball is projected horizontally ,we will get an right angled triangle by this and also given angle of 45.this can be written as

tan 45 = opp/adjacent=80/x

then we get x=80m=h then use time of flight formula is Sqrt(2h/g) by substituting the values  we get time  4sec

initial velocity,    x=ut

80 =u(4) we get u=20m/s.

and thats it ,do practice more so that you solve this kind of problems easily .

All the best!!