I realise how retarded i am
if aspect ratio is 16:9 and diognally is 61 cm then
61*9/square root of (16^2+9^2) = 29.9059356176
and then you can simply find out width by just multiplying it with 16/9*29.9059356176 = 53.1661077646
I am actually retarded and need to go back to middle school......