Answer:
B. +5.75 m/s
Explanation:
When there are two bodies, a and b, whose velocities measured by a third observer (in this case, the ground) are [tex]V_a[/tex] and [tex]V_b[/tex] respectively, the relative velocity of B with respect to A is given by:
[tex]V_{ba}=V_b-V_a[/tex]
Thus, the velocity of the girl relative to the lawnmower is:
[tex]V_{ba}=6.5\frac{m}{s}-0.75\frac{m}{s}\\V_{ba}=5.75\frac{m}{s}[/tex]
