You could use a method called cross products to solve this proportional equation. What you would do is multiply 3.4*7.7 =2.17*x
If multiplied correctly, your equation should be 26.18 = 2.17x
You are trying to get the variable alone on one side. To do this, you have to undo the multiplication of the coefficient. The opposite of multiplication is division, so you would divide both sides by 2.17.
If done correctly, your equation should now be x=12.064516129
However, this can be rounded to x= approximately 12.066