The function for the trapezoid area is:
A(x)=(B+b)*h/2
where B and b are the bases and h is the height.
With the given data: h=10 B and b =7 and x (it may vary which one is bigger)
So that function becomes:
A(x)=(7+x)*10/2
A(x)=(7+x)*5
So if you want the inverse function, you have to operate to find x:
A(x)/5=7+x
A(x)/5-7=x
So the new function is:
x(A)=A/5-7