solve a=1/2(b+c)h for c
a=(1/2(b+c))(h)1/2bh+1/2ch=a1/2bh+1/2ch+−1/2bh=a+−1/2bh1/2ch=−1/2bh+a1/2chh/2=−1/2bh+ah/2c=−bh+2a / h