Can anyone pls help me with this, I don't even know where to start? I can also give Brainliest

The interior angles of a parallelogram will always equal 360. The interior angles of a triangle are 180 if it is a equilateral triangle then the angles are 60 degrees each. You know 2 sides of parallelogram. The 100 degree and 90 degree. You can find the one opposing the x because it’s on a straight line and a straight line is 180 degrees if a triangle is 60 degree angle then the opposing angle to complete the straight line must be 180-60 so you get 120. You now know 3 sides. 120, 90 and 100
They should add up to be 360 so you can set up an equation like so
120+90+100+x=360
And x = 50

It's like discovering the answer to a puzzle. the purpose in solving an equation is to find the value or values of the variable that makes it a true statement. any value of the variable that makes the equation true is called a solution to the equation.