Answer
(a) The function is $p=15+0.434d$ where $p$ is in lb/in$^2$ and $d$ is in feet.
(b) The depth is $195.85$ feet.
Work Step by Step
(a) This function is linear. The pressure starts from $15$ lb/in$^2$ and increases by $4.34$ lb/in$^2$ for every ten feet of descend. If we measure the depth $d$ in feet we would have that the pressure increases for $4.34/10=0.434$ for every foot of descend:
$$p=15 + 0.434d.$$
(b) We will put $100$ lb/in$^2$ for pressure into the equation from part (a):
$$100=15+0.434d\Rightarrow 0.434d=100-15=85$$ and this gives
$$d=\frac{85}{0.434} = 195.85$$
in feet.
