The area of a trapezoid...

I need help on this math question, Im confused and i don't understand it ethier.

*(If you can, may you please write how you got your answer? Its ok if you cant.)*
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The area of a trapezoid is given by

A=1/2h(a+b)

where "h" is the height of the trapezoid, and "a" and "b: are the lenghts of the upper and the lower base, respectively.

Solve for "b"

ok first multiply by 2 so equation becomes
2A=h(a+b)
then divide by h so equation becomes
2A/H=a+b
finally minus a so final equation is
2A/H-a=b

A = 1/2h(a+b)
A = (1/2)ha + (1/2)hb (by using the distributive property)
A - (1/2)ha = (1/2) hb
2(A - (1/2)ha) / h = b

erwagaerhg

2A = ha + hb
b = (2A-ha)/h