How Can I Solve This 2D Kinematics Problem?

[praecox]

Homework Statement

A moderate wind accelerates a pebble over a horizontal xy plane with a constant acceleration = (4.60 m/s2) + (7.00 m/s2). At time t = 0, the velocity is (4.4 m/s). What are magnitude and angle of its velocity when it has been displaced by 11.5 m parallel to the x axis?

Homework Equations

rf = (x_i + V_xit + 1/2 a_xt²) i + (y_i + V_yit + 1/2 a_yt²) j
... I think

The Attempt at a Solution

I missed a day of class due to the flu, and our textbook sucks at explaining. I think the variables should be:
a_x = 4.6
a_y = 7
v_xi = 4.4
v_yi = 0
x_i = 0
y_i = 0
x_f = 11.5
So all I need to solve for is time, yes?
I tried to use a 1D equation to find t, since we have the V_i and x_f.
I plugged the x values into:
x_f - x_i = V_xit + 1/2 a_xt².
11.5 - 0 = 4.4t - 2.3t²
t² + 1.91t + (0.91) = 5 + 0.91 (divided by 2.3 and then completed the square)
(t + 0.96)² = 5.91
t + 0.96 = 2.43
t = 1.47 sec

When I plug in 1.47 as the time though, and then take the magnitude of the r_f vector, I get 13.7 m, but my online homework says the answer is 15.2.

I've tried a few other equations, but I just can't seem to get it... and that's just the first half of the problem. I'm not even sure where to start to find the angle. :/

 

[Thaakisfox]

It seems that you changed a sign. it should be 4.4t+2.3t^2 and you wrote -2.3t^2

 

[praecox]

Oh, yes. That was just a typo. when I did the calculation, I did it with 11.5 - 0 = 4.4t + 2.3t².