Placing the expression into graphing form:
y = 3x2 + 16x + 4
y - 4 = 3x2 + 16x Place the 4 on the other side
y - 4 = 3(x2 + 16/3x ) Factor out the 3
y - 4 + 64/9 = 3(x2 + 16/3x + 64/9) Complete the square
y + 28/9 = 3(x2 + 16/3x + 64/9) Simplify
y + 28/9 = 3(x + 8/3)2 Factor
y = 3(x + 8/3)2 - 28/9 Rewrite
There is still a little left for you to do ...