hi,

see there is some mathematics operator is missing between -3 and 4 , which has multiplied with m, I  am guiding you how to solve this,   you need to simplify the first equation, then find either m or n, now put the value in  the equation of 3m+7n, YOU may find the value of one unknown number , then again do the same thing find other unknown value, then put the value to m and n in equation again, you may get the final answer.