v = 0 m/s (at maximium height velocity is 0 m/s).
a = -9.8 m/s^2 (-ve because if u = +45 m/s then downwards is the negative direction).
v = u + at. Substitute the data. Solve for t.
You give no idea what you've been taught - I'm assuming you have met the uniform straightline motion formulae .....
It looks like you might need to go back and review your class notes and/or textbook.