T_{n} = a + (n-1)d
\therefore T_{n} = 2.9 + (n-1)\times 0.33
$n is the year, so$\,\,n=1\,\,$is the first year, etc$
$So in the$\,\, n^{th}\,\,$year, the diameter is$\,\, 19.4
\therefore 19.4 = 2.9 + (n-1)\times 0.33
$Solving yeilds:
n = 51
$The age of the tree when it was 19.4cm was 51...