x/28= 3/7 >>>28:7=4 >>>amplifici pe 3/7 cu 4 = 12/28
45/x=3/7>>>>45:3=15>>>amplifici cu 15 pe 3/7 = 45/105
2x+1/21=3/7>>>21:7=3>>>amplifici cu 3 pe 3/7=9/21>>> 2x+1=9>>2x=8>>x=4
51/38+x²=3/7>>>>51:3=17>>>amplifici cu 17 pe 3/7=51/119 >>>>38+x²=119
x²=81>>>x²=9²>>>x=9