Find the missing side length

You can let x = the length of the short side. Then the length of next largest side would be x+2 and the length of the largest side would be x+4. These sides add up to 195 mm. You can set up the equation to find x, the length of the short side. 
 Simplify and solve for x.
 Subtract 6 from both sides of the equation.
 Divide both sides by 3.
 
The length of the short side is 63 mm
The length of the middle side is x+2 = 63+2 = 65 mm
The length of the long side is x+4 = 63 + 4 = 67 mm 
Check: 63+65+67 = 195 mm