After 2 seconds the particle will be in position
[tex]x(2)=10+12\cdot 2^2=10+12\cdot 4=10+48=58[/tex]
After 5 seconds the particle will be in position
[tex]x(5)=10+12\cdot 5^2=10+12\cdot 25=10+300=310[/tex]
So, the particle will travel
[tex]310-58=252[/tex]
meters in 3 seconds, for an average speed of
[tex]\dfrac{252}{3}=84[/tex]
