Forgive me if this is too naive but isn't it as simple as saying that 2A = E + ? (where ? is the lower unknown angle), hence ? = 2A - E.
With this quantity known, A = 18.4 + ? (where ? is the upper angle...you really ought to use different symbols 
hence ? = A - 18.4.
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, light sensor, potentiometers, pushbuttons