A particle leaves the origin with a celocity of 7.8 m/s in the positive y direction and moves in the xy plane with a constant acceleration of (3.5i - 2.8j) m/s^2. At the instant the particle moves back across the x axis (y=0), what is the calue of its x coordinate?

I'm a beginner and am stuck. I have v= 7.8 m/s; a = (3.5i - 2.8j)m/s^2; y=0 and I need to find x. Where do I go from here?:confused: