First, draw a diagram (unless it is provided).
Then draw an FBD (free-body diagram for the beam)
Consider the conditions of equilibrium ( ∑ F = 0, ∑ M = 0 ).
Solve for unknowns.
Assuming the wall support is hinged (else insufficient information to solve).
We denote the beam ABC of length 2L, with a weight of 981 N acting at its CG at B.
The cable is attached at C at 45 degrees with the horizontal, with a tension of T newtons.
Sum vertical forces: we cannot sum vertical forces without knowing the vertical reaction at the wall support A.
Next, take moments about A (counter-clockwise positive)
T*(2Lsin(45)-981L=0
solve for T
T=981L/(2Lsin(45)=981/sqrt(2)=693.7 N