$4-18$ A steady, incompressible, two$-$dimensional velocity field is given by the following components in the $xy-$plane:

$u=1\mathrm{.}85+2\mathrm{.}05x+0\mathrm{.}656y$

$u=0\mathrm{.}754-2\mathrm{.}18x-2\mathrm{.}05y$

Calculate the acceleration field $($find expressions for acceleration components $a_x$ and $a_y),$ and calculate the acceleration at the point $(x,y)=(-1,3).$ Answers: $a_x=1\mathrm{.}51,a_y=2\mathrm{.}74$