The answer is 3(x+5)^2
Factor out 3 from the original equation
3(x^2+10x+25)
Then find two numbers that multiply to give 25 and add to give 10. The two numbers are 5 and 5
This gives...
3(x^2+5x+5x+25)
Factorise the expression in the brackets
3(x+5)(x+5)
Combine like terms
3(x+5)^2
