The focal length is positive for converging lens (convex) and mirrors (concave).  It is negative for diverging lens and mirrors.

The object distance is positive and we are solving for d_i

d_i = ( f ^-1  -  d_o ^-1 ) ^-1 =  

d_i = ( (10.0 cm) ^-1 - (25.0 cm) ^-1 ) ^-1 

d_i = 16.7 cm   since it is positive, it is a real image on the other side of the lens.

h_i          d_i
---- = - ----- 
h_o         d_o 

    
          d_i h_o           16.7 cm x 5.00 cm
h_i = - -------- =  - --------------------------
            d_o                    25.0 cm

    
h_i  =  -  3.34 cm

The negative sign shows the the image will be inverted.