Say you want to study the effect of age on income. You would control for other variables that could effect income (for example gender, race etc...). What you are really doing is holding those variables constant so you can see if there actually is a relationship between age and income. Controling makes your findings more powerful and 'true'.
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Use a variable to represent the unknown. 'Translate' the words to math symbols and write an equation to solve. Solve the equation. Check.
If the figures in the table are exact and without measurement error then take any two of the points (x1, y1) and (x2, y2) and use these to form the linear relation y - y1 = ((y2 - y1)/(x2 - x1))(x - x1) If, however, you suspect that the values in the table do not exactly follow a linear relationship then use linear regression for which formulae are provided in wikipedia.
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Many real life physics problems are parabolic in nature. Parabolas can be shown as a quadratic equation. If you have two variables then usually you can use the equation to find the best solution to a problem. Also, it is a beginning in the world of mathematical optimization. Some equations use more than two variables and require the technique used to solve quadratics to solve them. I just ran an optimization of 128 variables. To understand the parameters I needed to set I had to understand quadratics.
There are several methods. 1. graphing, then find the intersection. 2. Substitution (take one equation and solve for one variable, substitute that into the 2nd equation) 3. Elimination. Arrange both equations in standard form, arrange so that the coefficients on one of the variables are the same and subtract the 2 equations. 4. Cramer's rule, use matrices to solve.