MATH HELP!!!! Plz help and thnks?

Question

Find the value of the variable and ST if S is between R and T.

1) RS=7a, ST=12a, RT=76

2) RS= 2x, ST=3x, RT=25

3)RS=3y+1, ST= 2y, RT= 21

Answer 1

1) 7a + 12 a = 76

19a=76

a=4

2) 2x+3x=25

5x=25

x=5

3) 3y+1+2y=21

5y + 1 = 21

5y=20

y=4

Answer 2

Overall knowledge: RS+ST=RT

With that said:

1) 7a + 12a= 76

     19a=76

      a=4

             ST=12a=12(4)=48

2) 2x + 3x = 25

      5x=25

       x=5

         ST=3x=3(5)=15

3) 3y+1+2y=21

     5y=20

     y=4

         ST=2y=2(4)=8

Hopefully that helps