The attached image is the rhombus in question. To start, a rhombus will have perpendicular bisectors. With that, we know that the section where these bisectors cross will create 90 degree angles. So now we have angles: x, (x-10), and 90.
A triangle will total 180 degrees in total. So if we add up all of these angles, it will total to exactly 180.
In other words: 90+x+(x-10)=180.
So, in this situation, we want to isolate the variables, and find the value of x. This is the easy part:
Here's that equation again 90+x+(x-10)=180
Subtract 90 from both sides 90-90+x+(x-10)=180-90
x+(x-10)=90
Add 10 to both sides x+x-10+10=90+10 x+x=100 2x=100
