Communicate Mathematical ideas How do you know whether on both sides or subtract on both sides when solving an equatic?Please don’t give me a link.

The angle of a triangle are given as 6x,(x-3), and (3x+7). find x then find the measures of the angles

How many 3-digit numbers can you write using only digits 1 and 7? (of course, digits can repeat.) how many 3-digit numbers can you write using only digits 1 and 7? (of course, digits can repeat.)

Which term describes the point where the perpendicular bisectors of the three sides of a triangle intersect?