=LOOKUP(A2,{3,6,7,8,9},{"F","D","C","B","A"}) looks up the value in A2 (45) in the first row of the array, finds the largest value that is less than or equal to it, and then returns the value in the last row of the array that's in the same column.
Input the formula: =LOOKUP(A3,{3,6,7,8,9},{"F","D","C","B","A"}). Press Enter to get the result.