Question 1200332
Find h if A  =  39;  a  =  4;  b  =  9

A= 1/2h(a+b)
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Sub what you're given.
39 = h*(4+9)/2
78 = h*13
etc
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PS  A= 1/2h(a+b) is ambiguous.
Could be {{{A= (1/2)h(a+b)}}}
or {{{A= 1/(2h(a+b))}}}