Question

360-357+354-351+...+300-297

Answer 1

$360-357+354-351+...+300-297\\\\=(360-357)+(354-351)+...+(300-297)\\\\=\underbrace{3+3+...+3}_{11}=11\times3=33$

Answer 2

$x=360-357+354-351+...+300-297=3+3+3+...+3=3n\\\\\ \ \ \ and \ \ \ 300=360-6\cdot(n-1)\ \ \ \Rightarrow\ \ \ 6n=66\ /:6\ \ \ \Rightarrow\ \ \ n=11\\\\x=3\cdot11=33$