Which is the next step in the following construction of an equilateral triangle?

Which will form a composite function?

The measures of two supplementary angles are (2x + 10)° and (8x + 10)°. what is the measure of the larger angle? a. 16° b. 44° c. 14° d. 138°

How do you figure out which order to do two step equations in? like which thing do you subtract/add/multiply/divide first? i understand that you do the opposite of whatever's shown on the opposite side, but when?