Answer:
![MN= \boxed{15}](https://img.qammunity.org/2022/formulas/mathematics/high-school/49sts2aqcc9qtni2oal4forg98op62d4lb.png)
Explanation:
refer the attachment
therefore Let,
according to the question
![\displaystyle (x - 3 ) (x + 13) =( x + 3 {)}^(2)](https://img.qammunity.org/2022/formulas/mathematics/high-school/uc32qidmhrdkfj9tes3fb82ag0pr3axx4v.png)
simplify square and Multiplication:
![\displaystyle {x}^(2) + 10x - 39 = {x}^(2) + 6x + 9](https://img.qammunity.org/2022/formulas/mathematics/high-school/j142sj2ydhzqdz6jns4u58bmepa6iva3lg.png)
cancel x² from both sides:
![\displaystyle 10x - 39 = 6x + 9](https://img.qammunity.org/2022/formulas/mathematics/high-school/kb27ub67zwm4oxam9goom1sjs5r2sbjvte.png)
cancel 6x from both sides:
![\displaystyle 4x - 39 = 9](https://img.qammunity.org/2022/formulas/mathematics/high-school/navdpo6e86f1m2d5hd7czo3eu5mvrdyx41.png)
add 39 to both sides:
![\displaystyle 4x = 48](https://img.qammunity.org/2022/formulas/mathematics/high-school/azr97op4kogrog8f4xqpl8btmrxun4qzkb.png)
divide both sides by 4:
![\displaystyle \boxed{x = 12}](https://img.qammunity.org/2022/formulas/mathematics/high-school/ldt07u4no0urcgpbfhtv6rduru4nr0gw7p.png)
given that,
![MN= x + 3](https://img.qammunity.org/2022/formulas/mathematics/high-school/trzbqfcp1scimg42asolau5lyxdcxk66xu.png)
substitute the got value of x:
![MN= 12 + 3](https://img.qammunity.org/2022/formulas/mathematics/high-school/xazfoyxzk3qmfkienth025hwj09josxa9j.png)
simplify addition hence,
![MN= \boxed{15}](https://img.qammunity.org/2022/formulas/mathematics/high-school/49sts2aqcc9qtni2oal4forg98op62d4lb.png)