book Maths Puzzle Numerical Ability

I'm having problems grasping the concept and proceedure to solving algebraic equations, I'd appreciate it if you walk me step by step through an example equation, that I may grasp

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in a chem class, 7 liters of a 4% silver iodide solution must be mixed with a 10% solution to get a 6% solution. How many liters are needed?

I tried : to make a chart
or .10x + .04{7 + X}= .06

I got a neg. number. Please help.
Find the perimeter of a right triangle with one side 2cm and the hypotenuse 7cm.