Answer:
120⁰
Explanation:
The angles of a quadrilateral always add up to 360⁰.
Think of a square: it has four 90⁰ angles, right? That's 360⁰.
So since you're given 3 of the angles, you just need to add them up and see how much more you need to get to 360⁰:
75 + 90 + 75 = 240⁰
You should be able to see from there that you need 120⁰ more to get to 360⁰. So that's the unknown angle.
But to do it with algebra looks like this:
75 + 90 + 75 + x = 360
Where x = the unknown angle. Solve for x:
x = 360 - (75 + 90 + 75) = 120⁰