Alan

Here's my alternative substitution:

As follows:

As follows:

This graph should help

As follows:

As follows:

The ground is at height y = 0, so set y to zero and solve the resulting equation for t using the quadratic formula.  You will get two answers, but only the positive value is meaningful here.

The two expressions must be equal when x = 3, so:

3 + 2 = 2*3^2 + 3*3 + a

so

5 = 27 + a

I'll leave you to find a from this.

Subtract x from both sides and add 5 to both sides.  What do you get?