Tall pea plants are dominant over short pea plants. if there are 200 short plants in the f2 generation from a cross that followed mendel’s methods, about how many plants will be tall in that generation?

When two true breeding plants breed they produce an F1 generation. The genotype for the true breeding parents would be TT and tt. Each of the offspring in the F1 generation would have the genotype of Tt and would have the phenotype of being tall, since tall is dominant. Next, we breed the plants of the F1 generation to produce the F2 generation. We know that the phenotype of the parents are both Tt. From this you can create a punnet square:
      T        t
T    TT     Tt
t     Tt       tt

TT is expressed as tall
Tt is expressed as tall 
tt is expressed as short
The punnet square shows you that 75% would be tall and 25% would be short.
The 200 short plants in the questions would be 25% of 800. Take the 200 away from 800 and you get approximately 600 that would be tall.