I hope you answered with more detail and explanation than you have posted here !
I have travelled via the ONC, HNC and HND route (and further), here is something that I wish had been explained to me right at the start [actually, it probably was explained - I just didn't perceive the message
] You are not being tested on your ability to remember equations; the exams are an assessment of your understanding and ability to apply your acquired knowledge.
What this means is: Writing down the correct numerical answer gets you possibly one mark, two marks if the examiner is feeling generous or looking for straws to grasp to get you up to the pass-mark.
How many marks were allocated to this question?
How many of those marks did you accumulate?
I see The Electrician has beaten me to an answer. I do not know the exam rubric, however, there are probably more marks to be awarded to the Electrician for demonstrating an understanding of the question than the verbaitim regurgitation of an equation, ... an equation which is wrong, ... consider the outcome of setting \$\alpha\$ to zero (it should resolve to the standard sinewave R.M.S. - but it doesn't!)
"I have travelled via the ONC, HNC and HND route ..." and yes, it is 'kin frustrating!
Edit. I think the equation should have been \$ V_s \sqrt{\frac{1}{\pi} \left( \pi - \alpha + \frac{\sin 2 \alpha}{2} \right)}\$ which would return a Vrms of 89.7V.