What is the simplified form of the expression?

Anyone know? will mark brainliest if correct

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?

An isosceles triangle has two sides of equal length, a, and a base, b. the perimeter of the triangle is 15.7 inches, so the equation to solve is 2a + b = 15.7. if we recall that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side, which lengths make sense for possible values of b? select two options.