, If the sequence is arithmetic, we use the formula: three times. Once for n=3. once for n=8, and once using the values for t1 and d. -------------------------- Substitute n = 3 Substitute 15 for t3 ------------------------- Substitute n = 8 Substitute 50 for t8 So you have this system of equations to solve: Solve that system by substitution or elimination and get and --------------------------------- Therefore becomes Edwin