# ReplaceAll error

i"m trying to replace theta1 and theta2 to x1,x2,x3,x4. What is wrong with this replace method?

• Please post code not images. Look here to figure out how. Once you do that we can help you. – b3m2a1 Dec 5 '17 at 21:52
• One obvious error is theta1'  means nothing. theta1'[t] does – Bill Watts Dec 6 '17 at 0:19
• i tried it using thetat1'[t] -> x2[t], same error occurs – Hussein Dec 6 '17 at 9:41