Answer:
x = 3
Explanation:
The smaller triangle and larger one are similar, so corresponding sides are proportional.
8/4 = (8 +(x -1))/5
10 = x +7 . . . . . . . . . multiply by 5 and simplify
3 = x . . . . . . . . . subtract 7
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Alternate solution
The short segments are also proportional:
(x -1)/(5 -4) = 8/4
x -1 = 2 . . . . simplify
x = 3 . . . . . . add 1