The triangles are similar. Set the two triangle's measurements equal to each other
8/10 = (x - 1)/(x + 2)
Cross multiply
8(x+ 2) = 10(x - 1)
Distribute the numbers
8(x) = 8x
8(2) = 16
10(x) = 10x
10(-1) = -10
8x + 16 = 10x - 10
Isolate the x. Subtract 16 from both sides, and 10x from both sides
8x (-10x) + 16 (-16) = 10x (-10x) - 10 (-16)
8x - 10x = -10 - 16
Simplify. Combine like terms
-2x = -26
Isolate the x. Divide -2 from both sides
-2x/-2 = -26/-2
x = -26/-2
x = 13
13 is your answer for x
hope this helps