I don’t know how to do this can someone help out

Answer:
Third side = 4
Step-by-step explanation:
Use Pythagoras theorem :
C = longer side, a = 10, b ?
[tex]c^2 = a^2 + b^2[/tex]
[tex](2 \sqrt{29})^2 = 10^2 + b^2\\\\4 \times 29 = 100 + b^2\\\\116 - 100 = b^2\\\\b^2 = 16 \\\\b = 4[/tex]