Angle1 +angle2+angle3=180(property of triangle)
Angle1+angel2=angle3(according to question)
Thus,
Angle3+angle3=180
2*angle3=180
Angle3=90
Means this is a right triangle
Now,we can use formula
Side1^2+side2^2=side3^2
Thus,
4*4+5*5=side3^2
Square root(36)=side3
Side3=6