$27\mathrm{.}7$ A particle has a velocity component in the $y-$direction of $v_y=8t,$ and an accelertion component in the $x-$direction of $a_x=4t^2.$ When $t=0sec,y=2ft,x=0,$ and $v_x=0.$ Calculate the magnitude of the velocity of the particle when $x=18ft.$

$($Ans$.v=34\mathrm{.}3\frac{ft}{sec})$