STEP 1: find side $ x $
To find side $ x $ use formula:
$$ x = \frac{ a - b } { 2 } $$After substituting $ a = 18 $ and $ b = 10 $ we have:
$$ x = \frac{ 18 - 10 } { 2 } $$ $$ x = \frac{ 8 } { 2 } $$ $$ x = 4 $$STEP 2: find side $ c $
To find side $ c $ use Pythagorean Theorem:
$$ h^2 + x^2 = c^2 $$After substituting $ h = 3 $ and $ x = 4 $ we have:
$$ 3^2 + 4^2 = c^2 $$ $$ 9 + 16 = c^2 $$ $$ c^2 = 25 $$ $$ c = \sqrt{ 25 } $$$$ c = 5 $$STEP 3: find perimeter $ P $
To find perimeter $ P $ use formula:
$$ P = a + b + 2 \cdot c $$After substituting $ a = 18 $ , $ b = 10 $ and $ c = 5 $ we have:
$$ P = 18 + 10 + 2 \cdot 5 $$$$ P = 28 + 10 $$ $$ P = 38 $$