15+(10-x)=15 would be your equation if that is what you are looking for.
ANSWER

and
e have
EXPLANATION


Let us make y the subject and call it equation (2)


We put equation (2) in to equation (1)



Simplify to get,


Divide both sides by 31,



We put this value in to equation (2) to get,


We collect LCM to obtain,


It’s 42 but Carrier the 2
No, because a trapezoid has four sides.
“A” is the answer i believe !
hope this helps