Question

sastika said, "let us study hard to get success"---- change the mode of narration

Answer 1

Your question : Sastika said, "let us study hard to get success".
Your answer : Sastika suggested that we should study hard to get success.

Answer 2

Sastika suggested that we should study hard to get success.

When changing a sentence from direct to indirect, it is very necessary that the word said is replaced with a more appropriate verb like asked, proposed, suggested, advised, etc.