samuel drives his bicycle east at a velocity of 10.0 m/s. he then increases his speed in the same direction to 25.0 m/s in 3.50 s. what is his average speed

Ave speed from when to when?

I'll assume during the 3.5 seconds.
x = 1/2(vi + vf)t
v(ave) = x/t = 1/2(vi + vf)
And why bother giving a direction if you're asking for speed?