this is very simply since QPT is similar to QRS then we can set this as QP/QR=PT/RS=QT/QS now solve for x when putting in the values of each line. for example QP is is 20 and QR is x this means our first fraction is 20/x, now our second one is 12/8 sicne Pt is 12 and Rs 8, and our last one is 20/x. now we have 20/x=12/8=20/x lets just say we have 12/8=20/x first we have to find a common denominator in this case it can be 8x so multiply your numerator times the number you used to get 8x in the denominator, once you do it you will get 12x/8x=160/8x, now we can cancel our denominator sicne it is the smae thing and be left with 12x=160, now divide both sides by 12 to get x-=160/12, now simplify lets divideit by 4 first and get x=40/3 can that be simplified any more?? I dont think so so your final answer should be x=40/3
Hope this helps