Casio fx-991EX classwiz calculator

A question is asking me to give an exact answer for tan theta, and i do tan(180-arcsin(2/3)) which i get as -0.894427191 but I cannot change it into fraction form using the S-D button as the fractional form from the mark scheme is (-2√5)/5 how would i convert this on my calculator as the values are the exact same

Not really your question (no calculator) but using a sketch of the angle/triangle, the opp is 2, hyp 3 so adj using pythagoras is sqrt(5) which pretty much gives your answer

the actual question isnt too complicated, this is the actual question

A triangle includes an obtuse angle theta. The two shorter sides of the triange have lengths 4cm and 7.5cm; the area of the triangle is 10cm^2. Find the exact value of tan theta

so i just used area = 1/2abSinC so i got 2/3 = Sin theta then as its an obtuse angle its between 90 and 180 so i did tan(180-theta) to get my answer but i cannot convert it to fraction form

Sounds about right. So
sin(C) = 2/3
and its obtuse means (quick sketch) youre in quadrant 2 with a right triangle sqrt(5) : 2 : 3 (using pythagoras), so tan of the acute angle is 2/sqrt(5) which means tan(C) = -2/sqrt(5) = -2sqrt(5)/5.

Alright thanks, is there no way for me to just get my calculator to convert the decimal into that fraction as i said before the S-D button wont work for that