pythagorean theorem
for a right triangle with legs (2 sides next to the the right angle) length a and b, and hyptonuse (longest side oposite right angle) is c
![a^2+b^2=c^2](https://img.qammunity.org/2019/formulas/mathematics/high-school/sjkaahpsyltp8n99but72kh8jxe05yu3k3.png)
we can see that the legs are 3cm and 7cm
so a=3 and b=7 (you can do a=7 and b=3 but it doesn't matter)
![a^2+b^2=c^2](https://img.qammunity.org/2019/formulas/mathematics/high-school/sjkaahpsyltp8n99but72kh8jxe05yu3k3.png)
![3^2+7^2=c^2](https://img.qammunity.org/2019/formulas/mathematics/middle-school/e34t3ppkgih57mly828zmq1fmzdjmyn5e9.png)
![9+49=c^2](https://img.qammunity.org/2019/formulas/mathematics/middle-school/v9aem9lovbaqwgdu7lcuq6w1mjqne73lzm.png)
![58=c^2](https://img.qammunity.org/2019/formulas/mathematics/middle-school/2livks03t1yisjmpe0drzw2jpnazbn8578.png)
sqrare root both sides
![√(58)=c](https://img.qammunity.org/2019/formulas/mathematics/middle-school/j515aprdpnqk2bv9i9qmnhiwe6c931iabm.png)
![c=√(58)](https://img.qammunity.org/2019/formulas/mathematics/middle-school/fss0mazyrmeht56cnvlzpxunmgorwypy1o.png)
answer is 4th option