Kicking a ball down a hill

[KindofSlow]

A ball is kicked horizontally at 15 m/s from atop a hill sloping downward at 45 degrees.
How long does it take for the ball to hit the ground?
My work:
x = Vsubx*t
y = 1/2*g*t^2 (I've made downward positive in the y direction.)
Since hill is 45 degrees, I've concluded x = y when the ball hits the ground.
4.9*t^2 - 15t = 0
Solving for t, I get 3.06 seconds,

The book says 5.1 seconds.
Any guidance regarding any errors I am making will be greatly appreciated.

Thank you

 

[KindofSlow]

I think I put this in the wrong section.
I've copied over to the Calculus section.
Apologies and thank you.

 

[soroban]

Hello, KindofSlow!

A ball is kicked horizontally at 15 m/s from atop a hill sloping downward at 45 degrees.
How long does it take for the ball to hit the ground?

My work:

x = v_xt
y = (1/2)gt² . (I've made downward positive in the y-direction.)

Since hill is 45^o, I've concluded x = y when the ball hits the ground.
. . 4.9t² - 15t .= .0

I get: .t = 3.06 seconds. . Right!

The book says 5.1 seconds. . The book is WRONG!

Could there be a typo?

The book is correct if the initial velocity is 25 m/s.

 

[KindofSlow]

Thank you very much Soroban.
The book is Giancoli's Physics for Scientists and Engineers - Custom edition for UC Berkeley.
I just double checked and no typo - probably the student they hired to solve the problems used 25 in error.
They don't show their work - they just have the answers in the back of the book.
Your assistance is extremely helpful.
Thanks again.