Algebra operations can be performed on equations with symbols, numbers, or measurements. An equation states that the right side of the equation is equal to the left side. The rules of manipulation of the two sides of an equation without changing the equation are: (a) you can multiply or divide both sides by the same thing, (b) you can add or subtract the same thing from both sides, (c) you can raise both sides to the same power, you can change both sides to or from any exponent. Also, you can substitute anything in an equation for something of equal value. The rules are easy, but becoming proficient takes practice. If you need the practice, there are many basic algebra textbooks and web sites. Most people need to actually DO the math in the textbook or web site rather than just reading about them.
One of the really big mistakes many novice calculators make is attempting to do too much between the ears before enough practice has been done. The best advice is that calculating with a pencil (or writing implement of any type) is much more reliable than calculating mentally. You may have heard of "back- of- the- envelope" calculations by scientists or engineers. Most people of science or technology understand that even something scratched on the back of an envelope is more reliable than mental calculation. Show your work, if not to your teacher, to yourself. Show the addition of the same thing to both sides. Show the cancellation of units. Show the stepwise conversion of what you know to what you need. Except for log and antilog work for pH calculations, the use of more complex algibraic operations in basic chemistry course is rare. The use of quadratics or complex factoring is rare.