Question 1209542
Which strategy depends on what he knows.  Upon appearance, substitution seems not the best choice but it can work.  


{{{system(3x+4y=25,4x-7y=-16)}}}


Elimination Method:
{{{system(12x+16y=200,12x-21y=-48)}}}


{{{system(12x+16y=200,-12x+21y=48)}}}

ADD:

{{{37y=248}}}

{{{y=248/37}}}

{{{y=6&26/37}}}
But recheck this part in case of mistakes in the work.