Answer
(a) $W = 1.26\times 10^5~J$
(b) $P = 1.46~watts$
Work Step by Step
(a) Note that $1~m^3 = 1\times 10^3 ~liter$
We can find the mass of the blood.
$m = (1.05\times 10^3~kg/m^3)(7.5~m^3)$
$m = 7875~kg$
We can find the work done by the heart each day.
$W = mgh$
$W = (7875~kg)(9.80~m/s^2)(1.63~m)$
$W = 1.26\times 10^5~J$
(b) We can find the heart's power output.
$P = \frac{W}{t}$
$P = \frac{1.26\times 10^5~J}{(24~hr)(3600~s/hr)}$
$P = 1.46~watts$